Pattern of follicular growth and resumption of ovarian activity in post-partum beef suckler cows

The ovaries of 18 post-partum beef suckler cows were examined daily, using ultrasound, from Day 5 post partum until a normal oestrous cycle was completed. Periods of growth and regression of medium-sized (5-9 mm) follicles were identified before one medium follicle became dominant (single large follicle greater than or equal to 10 mm). The mean (+/- s.e.m.) number of days from parturition to detection of the first post-partum dominant follicle was 10.2 +/- 0.5. The first post-partum dominant follicle ovulated in 2/18 (11%) cows. The interval from calving to first ovulation (mean +/- s.e.m. = 35.9 +/- 3.3 days) was characterized by the growth and regression of a variable number (mean = 3.2 +/- 0.2; range 1-6) of dominant follicles. The maximum diameter of the dominant follicle increased as the cows approached first ovulation (P less than 0.05). Behavioural oestrus was not detected in 16/18 (89%) cows at first ovulation. Following first ovulation, the length of the subsequent cycle was short (mean = 9.7 +/- 0.5 days; range 8-15 days) in 14/18 (78%) cows and was characterized by the development and ovulation of a single dominant follicle. During oestrous cycles of normal length (mean = 20.6 +/- 0.5 days; range 18-23 days) one (N = 2), two (N = 7) or three (N = 8) dominant follicles were identified. The growth rate, maximum diameter or persistence of non-ovulatory dominant follicles before first ovulation or during oestrous cycles were not different (P greater than 0.05). These data show that, in beef suckler cows, follicular development and formation of a dominant follicle occur early after parturition and the incidence of ovulation of the first dominant follicle is low. The number of dominant follicles that develop before first ovulation is variable; first ovulation is rarely associated with oestrus and short cycles are common after first ovulation. It is concluded that prolonged anoestrus in post-partum beef suckler cows is due to lack of ovulation of a dominant follicle rather than delayed development of dominant follicles.     

Behavioral, follicular and gonadotropin changes during the estrous cycle in donkeys

Sexual behavior, follicular development and ovulation, and concentrations of circulating gonadotropins during the estrous cycle were studied during the summer in 7 jennies. Mean behavioral estrous length was 6.4 +/- 0.6 days (mean +/- SEM, n=19; 5.6 +/- 0.5 days preovulatory and 0.8 +/- 0.2 days post-ovulatory). Mean diestrous length was 19.3 +/- 0.6 days (n=14). Females in estrus typically showed posturing, mouth clapping, clitoral winking, urinating and tail raising. Mouth clapping began approximately one day sooner and lasted approximately one day longer than winking and tail raising, so that the total duration of clapping was significantly greater than for the other two signs. Follicular changes and concentrations of gonadotropins were determined for 14 estrous cycles (2 per jenny). The follicular end points [diameter of the largest follicle and number of large (>25 mm), medium (20-24 mm), and small follicles (<20 mm)] showed a significant day effect. The diameter of the largest follicle and the number of large follicles began to increase significantly 7 days prior to ovulation with a maximum value the day before ovulation. Medium follicles reached a maximum number 4 days prior to ovulation, and small follicles decreased significantly prior to ovulation. After ovulation, all follicular end points, except the number of small follicles, remained low for the next 12 days. Mean values of FSH were low during estrus and high during diestrus with 2 significant peaks, one 3 days and one 9 days after ovulation. In contrast, mean levels of LH were low during diestrus and high during estrus with a maximum value the day after ovulation. The LH profile showed a more prolonged gradual increase prior to ovulation, than that which has been reported for ponies and horses.     

Pulsatile administration of gonadotropin-releasing hormone advances ovulation in cycling mares

Cycling standardbred mares were infused with saline or 20 micrograms gonadotropin-releasing hormone (GnRH) in a pulsatile pattern (one 5-sec pulse/h, 2 h or 4 h) beginning on Day 16 of the estrous cycle. Although serum concentrations of luteinizing hormone (LH) increased significantly earlier in all three GnRH-treated groups (within one day of the initiation of infusion) compared to saline-infused controls, there were no differences in peak periovulatory LH concentrations among treatments (overall mean +/- SEM, 8.98 +/- 0.55 ng/ml). The number of days from the start of treatment to ovulation was significantly less in mares infused with 20 micrograms GnRH/h (mean +/- SEM, 2.9 +/- 0.6 days after the initiation of treatment, or 18.9 days from the previous ovulation; N = 7) compared to mares treated with saline (5.9 +/- 0.3 days, or 21.9 days from previous ovulation; N = 7) or 20 micrograms GnRH per 4 h (5.4 +/- 0.9 days or 21.4 days from previous ovulation; N = 5). Although mares infused with 20 micrograms GnRH/2 h ovulated after 4.3 +/- 0.7 days of treatment (Day 20.3; N = 7), this was not significantly different from either the control or 20 micrograms GnRH/h treatment groups. Neither the duration of the resulting luteal phase nor the length of the estrous cycle was different between any of the treatment groups (combined means, 14.7 +/- 0.2 days and 21.3 +/- 0.4 days, respectively). We conclude that pulsatile infusion of GnRH is effective in advancing the time of ovulation in cycling mares, but that the frequency of pulse infusion is a critical variable.     

Vaginal pH during estrus in mares

Vaginal hydrogen ion concentration of Saddlebred mares was measured throughout the behavioral estrous period. The mean pH on the day of ovulation was significantly (P=<.01) lower than on all other tested days of estrus. Follicular development and vaginal pH values in pony mares at time of slaughter were highly correlated. A significant decrease in vaginal pH values, determined through a series of carefully monitored measurements during estrus, was found to be characteristic and indicative of ovulation.     

Day of cycle affects changes in equine intrauterine pressure in response to teasing

Oxytocin is released in response to teasing during both estrus and diestrus in mares, and at least during estrus, teasing results in an increase in electromyographic activity in the uterus. Exogenous oxytocin causes an increase in intrauterine pressure and prior studies have shown that this response is correlated to the day of the estrous cycle. To determine if teasing causes an increase in intrauterine pressure and if this response varies by day of the cycle, intrauterine pressure was measured while mares were teased with a stallion 2 days before ovulation, on the day ovulation was detected and 2 days after ovulation. A significant increase in intrauterine pressure was observed in response to teasing both 2 days before ovulation and on the day of ovulation, when plasma concentrations of progesterone were low. No significant increase in intrauterine pressure was observed in response to teasing 2 days after ovulation when progesterone concentrations were elevated. Management practices that include teasing or stallion exposure may be beneficial in stimulating uterine clearance mechanisms in mares during the preovulatory period.     

Oestrus cycle characteristics and prediction of ovulation in Catalonian jennies

The Catalonian donkey breed is in danger of extinction, and much needs to be learned about the reproductive features of its females if breeding and conservation programmes are to be successful. This study reports the oestrous behaviour, oestrus cycle characteristics and dynamic ovarian events witnessed during 50 oestrous cycles (involving 106 ovulations) in 10 Catalonian jennies between March 2002 and January 2005. These jennies were teased, palpated transrectally and examined by ultrasound using a 5 MHz linear transducer-daily during oestrus and every other day during dioestrus. Predictors of ovulation were sought among the variables recorded. The most evident signs of oestrus were mouth clapping (the frequent vertical opening and closing of the mouth with ears depressed against the extended neck) and occasional urinating and winking of the vulval lips (homotypical behaviour). Interactions between jennies in oestrus were also recorded, including mounting, herding/chasing, the Flehmen response, and vocalization (heterotypical behaviour). Nine jennies ovulated regularly throughout the year; one had two anovulatory periods (54 and 35 days). The length of the oestrus cycle was 24.90 +/- 0.26 days, with oestrus itself lasting 5.64 +/- 0.20 days (mean +/- S.E.M.) and dioestrus 19.83 +/- 0.36 days. The incidence of single, double and triple ovulations was 55.66% (n=59), 42.45% (n=45) and 1.89% (n=2), respectively. No significant difference was seen in the number of ovulations involving the left and right ovaries (52.63% [n=70] compared to 47.37% [n=63] respectively; P>0.05). The mean interval between double ovulation was 1.44 +/- 3.98 days. The mean diameter of the preovulatory follicle at day -1 was 44.9 +/- 0.5 mm; the mean growth rate over the 5 days before ovulation was 3.7 mm/day. Data on preovulatory changes in oestrous behaviour, follicle size, follicle texture, the echographic appearance of the follicle and uterus, and uterine tone were subjected to stepwise logistic regression analysis to detect predictors of ovulation. The logit function showed the best predictors to be follicle size, follicular texture and oestrous behaviour. Certain combinations of these three variables allow the prediction of ovulation within 24 h with a probability of >75%.     

Ultrasound and endocrine evaluation of the ovarian response to PGF2alpha given at different stages of the luteal phase in ewes

The purpose of this study was to evaluate the ovarian response of ewes to two treatments with PGF2alpha using transrectal ovarian ultrasonography and hormone measurements. Fifteen milligrams of PGF2alpha was given to six cyclic Western White Face (WWF) ewes early in the estrous cycle (Days 4 to 7) and to six late in the cycle (Days 10 to 12 after ovulation), and a second treatment was given 9 days after the first. Ultrasound scanning and blood sampling started 7 days prior to the first PGF2alpha treatment and ended 10 days (scanning) or 19 days (blood sampling) after the second PGF2alpha treatment, for both groups of ewes. Mean ovulation rate (2.6 +/- 0.7) did not differ significantly between the ewes first treated early or late in the cycle, or after the first or second treatments with PGF2alpha. The time from treatment to ovulation was longer in ewes first treated early (4.0 +/- 0.3 days) compared to late (2.8 +/- 0.4 days) in the cycle (P < 0.05). Both the number of ovulations (range: 0-7) and time from treatment to ovulation (range: 1-9 days) were highly variable. This variability appeared to be due to the extension of the life span of ovulating follicles that emerged prior to PGF2alpha administration and also ovulation of some follicles that emerged after treatment. When results for first and second treatments were pooled, the total number of follicles > 5 mm in diameter on the day of treatment that failed to ovulate in response to PGF2alpha was higher in ewes first treated early (0.8 +/- 0.2/ewe) compared to late (0.3 +/- 0.2/ewe) in the cycle (P < 0.05). The proportion of detected luteal structures relative to the number of ovulations was lower in ewes first treated early compared to late in the cycle (60 and 86%, respectively; P < 0.05). Disruption of ovulatory follicle dynamics and normal luteogenesis, and variability in the timing of ovulation after PGF2alpha treatments could all contribute to poor or variable fertility when prostaglandins are used for estrus synchronization.     

Development in the cyclic hamster of refractoriness to the superovulatory action of anti-LH serum

Hamsters injected s.c. on the day of ovulation (Day 1) with 100 microliters equine anti-bovine LH serum ovulated 28 eggs at the end of a 5-day cycle. When a second injection of anti-LH serum was administered 4-93 days later, the animals did not superovulate and had normal 4-day cycles. Injection of 100 microliters normal rabbit serum (NRS) on Day 1 followed 14 days later by anti-LH serum resulted in the ovulation of 32 ova whereas a priming injection of 100 microliters normal horse serum (NHS) followed by anti-LH serum resulted in the ovulation of only 18 ova. When hamsters were injected on Day 1 with anti-LH serum, NHS or NRS and then with anti-LH serum in the 4th cycle, high titres of free antibodies to LH were present on Days 2-4 only in the animals treated with NRS; these hamsters ovulated a mean of 35 ova. These experiments suggest that the hamster rapidly forms antibodies to equine immunoglobulins, thus preventing a second injection of anti-LH serum from inducing superovulation.     

Cytologic, hormonal, and ultrasonographic correlates of the menstrual cycle of the New World monkey Cebus apella

Few reports on the reproductive physiology of Cebus apella have been published. In this study we characterized menstrual cycle events by means of vaginal cytology, ultrasonography (US), and hormonal measurements in serum during three consecutive cycles in 10 females, and assessed the probability that ovulation would occur in the same ovary in consecutive cycles in 18 females. The lengths and phases of the cycles were determined according to vaginal cytology. Taking the first day of endometrial bleeding as the first day of the cycle, the mean cycle length +/- SEM was 19.5+/-0.4 days, with follicular and luteal phases lasting 8.2+/-0.2 and 11.3+/-0.4 days, respectively. The follicular phase included menstruation and the periovulatory period, which was characterized by the presence of a large number of superficial eosinophilic cells in the vaginal smear. The myometrium, endometrium, and ovaries were clearly distinguished on US examination. During each menstrual cycle a single follicle was recruited at random from either ovary. The follicle grew from 3 mm to a maximum diameter of 8-9 mm over the course of 8 days, in association with increasing estradiol (E(2)) serum levels (from 489+/-41 to 1600+/-92 pmol/L). At ovulation, the mean diameter of the dominant follicle usually decreased by >20%, 1 day after the maximum E(2) level was reached. Ovulation was associated with an abrupt fall in E(2), a decreased number of eosinophilic cells, the presence of leukocytes and intermediate cells in the vaginal smear, and a progressive increase in progesterone (P) levels that reached a maximum of 892+/-65 nmol/L on days 3-6 of the luteal phase. The menstrual cycle of Cebus apella differs in several temporal and quantitative aspects from that in humans and Old World primates, but it exhibits the same correlations between ovarian endocrine and morphologic parameters.     

Time of ovulation in ewes after treatment with a prostaglandin F-2alpha analogue

Treatment of 18 cyclic Clun Forest ewes with two i.m. injections of ICI 80,996, given 9 days apart and without reference to stage of the oestrous cycle, synchronized ovulation in all ewes at a mean time interval of 73.1 +/- 1.6 (s.e.m.) h from the second injection. The interval from the LH peak to ovulation was 22.6 +/- 0.7 h and this is comparable to previously reported gigures for a natural oestrus.     

Time of ovarian follicular recruitment in cyclic pony mares

An experiment was carried out on pony mares to establish the time of the oestrous cycle at which ovarian follicles are recruited for ovulation. In one group (n=7), the cycle was interrupted at the preovulatory stage by removing the preovulatory follicle; in another group (n=13) the cycle was interrupted at day 6 of the luteal phase by inducing luteolysis with a prostaglandin injection (PG). In a subgroup (n=7) of those given PG, the ovary not bearing the corpus luteum was removed at the time of injection. A further group (n=6) served as surgical controls. The interval to the next ovulation and blood concentrations of FSH were observed. Anaesthesia alone induced in preovulatory mares was followed by normal ovulation 2.5+/-1 days later. Removal of the preovulatory follicle delayed the next ovulation (14.6+/-2.1 days; P < 0.01). Following PG injection, the interval to ovulation was similar regardless of whether an ovary was removed (12.8+/-4.3 days) or not (10+/-4.1 days). This similarity occurred despite a large and prolonged rise in plasma FSH levels that occurred only in the hemiovariectomized group. In addition, the intervals found after PG injection did not differ from those found after ablation of the preovulatory follicle. These observations indicate that 1) in the presence of the early active corpus luteum or dominant follicle, follicles grow to a similar stage of development; 2) recruitment of the follicle due to ovulation occurs 12 to 14 days before ovulation; 3) limiting new follicular growth to one ovary does not affect the time course to ovulation; and 4) prolonged high FSH levels do not alter the time course or ovulation rate.     

Response of the 1-5 day-aged ovine corpus luteum to prostaglandin F2alpha

The hypothesis that, in the ewe, prostaglandin (PG) F2alpha administration on day 3 after ovulation is followed by luteolysis and ovulation was tested using 24 animals. The ewes were treated with a dose of a PGF2alpha analogue (delprostenate, 160 microg) on days 1 (n=8), 3 (n=8) or 5 (n=8) after ovulation, was established by transrectal ultrasonography. Daily scanning and blood sampling were performed to determine ovarian changes and progesterone serum concentrations by radioinmunoassay. The treatment induced a sharp decrease of progesterone concentrations followed by oestrus and ovulation in all ewes treated on days 3 and 5 and in one ewe treated on day 1 (8/8, 8/8, 1/8; P<0.05). Seven ewes treated on day 1 did not respond to PGF2alpha treatment and had an inter-ovulatory cycle of normal length (17.4 +/- 0.5 days). However, the profile of progesterone concentrations during the cycle of these ewes was delayed 1 day (P<0.05) compared with a control cycle. The overall interval between PGF2alpha and oestrus for the 17 responding ewes was 42.4 +/- 2.3 h. In 15 of these ewes the ovulatory follicle was originated from the first follicular wave and the ovulation occurred at 60.8 +/- 1.8 h after PGF2alpha treatment. The other two responding ewes ovulated an ovulatory follicle originated from the second follicular wave between 72 and 96 h after treatment. These results support the hypothesis and suggest that refractoriness to PGF2alpha of the recently formed corpus luteum (CL) may be restricted to the first 1-2 days post-ovulation.     

Non-invasive translational Cynomolgus model for studying folliculogenesis and ovulation using color Doppler ultrasonography

Background In women, different events of folliculogenesis can be measured and evaluated using ultrasound (US) technology. The availability of a non‐invasive translational non‐human primate model to study these processes would represent a major contribution to further advance R&D efforts toward novel therapies in assisted reproduction. Methods In our study, follicular growth and ovulation was measured in six cyclic Cynomolgus monkeys using abdominal Doppler US. Results The mean follicular diameter on cycle day ?6 (cycle day 0 = day of ovulation) was 3.7 mm that increased to 6.8 mm on cycle day ?1. After ovulation, the mean diameter decreased to 4.6 mm, confirming ovulation. The mean percentage of follicular size reduction after ovulation was 31%. Conclusion Ultrasonography in combination with color‐flow Doppler imaging was shown to be a useful, non‐invasive translational method to measure ovarian follicular growth and occurrence and timing of follicular rupture in Cynomolgus monkeys.     

Time of ovarian follicle selection during the porcine estrous cycle

Two experiments were conducted to: 1) determine the time during the procine estrous cycle when compensation in ovulation rate after unilateral ovariectomy (ULO) ceases to be complete, 2) compare the follicle selection process in gilts selected for high ovulation rate with unselected control gilts and 3) determine the number of follicles on the right ovary at various stages of the estrous cycle. Experiment I included 25 crossbred gilts, while Experiment II included 17 gilts selected for high ovulation rate and 16 unselected control gilts. The right ovary was removed via a mid-ventral laparotomy on either day 13, 15, 17 or 19 of the cycle. In Experiment I, compensation in ovulation rate ceased between days 13 and 15; whereas, in Experiment II, cessation occurred between days 15 and 17. Selected and control gilts responded alike to ULO, indicating similarity in the follicle selection process. Follicle numbers in the right ovary showed a general decline, especially between days 17 and 19, indicating that atresia was occurring during the follicular phase. The results indicate that the selection of ovarian follicles for ovulation at the ensuing estrus occurs before day 17 of the porcine estrous cyle.     

Increasing ovulation rate and lambing rate in sheep by treatment with a steroid enzyme inhibitor

Treatment of ewes with a 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD) inhibitor (Epostane) resulted in a significant increase in both ovulation rate and in the mean number of lambs per ewe lambing. The progestagen sponge plus 3 beta-HSD inhibitor treatment also caused a significant increase in oestrous cycle duration of approximately 1.5 days. Treatment of ewes with the 3 beta-HSD inhibitor caused a significant decrease in peripheral progesterone concentrations, which were reduced even further when 3 beta-HSD inhibitor treatment was given to ewes after insertion of a progestagen sponge. However, mean oestradiol concentrations were significantly higher in the two treatment groups, both at the end of the luteal phase and during the follicular phase of the oestrous cycle. These results demonstrate that ovulation rate and the production of lambs per ewe lambing can be significantly increased by 3 beta-HSD inhibitor treatment.     

Concentrations of steroids in the utero-ovarian vein blood,serially collected from the two sides of individual baboons,during the follicullar phase

The purpose of this study was to determine and compare the follicular phase steroid hormone secretion into the utero-ovarian vein by the ovary with a dominant follicle and the contralateral ovary in the same baboon. Serial utero-ovarian vein blood from both sides was collected in 25 baboons by the use of a laparoscope on alternate days, starting on day 1 or 3 of the cycle and continuing through 2 to 3 days post-ovulation. Approximately 3–4 days before the day of expected ovulation, samples were collected at 8-hr intervals. Steroids estradiol (E₂) and progesterone (P) were measured in all utero-ovarian vein plasma by radioimmunoassay. In the peripheral plasma, E₂, P, LH, and FSH measurements were carried out. Concentrations of steroids were significantly higher on the side of the ovulating ovary by day 5 before ovulation. Individual plots however, indicated that some baboons may establish the dominant side as early as day 11 before ovulation. The preovulatory gonadotropins had a differential effect on the two ovaries. For example, E₂ values on the ovulatory side ovary declined after increases in LH/FSH, whereas on the contralateral side these values had increased. Both sides showed increases in the level of P with the increases in LH. The mean interval from E₂ peak to LH peak was 24 hrs and LH peak to ovulation was 24 hrs.     

Probably never human. Review of chimpanzee material culture: Implications for human evolution,by W.C. McGrew. Cambridge,U.K., Cambridge University Press, 1992, 277 pp, $79.95, cloth

Pregnanediol-3α-glucuronide (PdG) was measured in the urine of six Goeldi's monkeys during pregnancy and the postpartum period. A stress-free, non-invasive urine sampling technique permitted frequent collection of urine from members of the breeding group. A comparison of the periovulatory profiles of PdG and estrone conjugates revealed close agreement. The day of ovulation was defined as that immediately preceding a 2-4 day period with two consecutive urine samples for which the PdG content was in excess of 0.20 μg/mg Cr and 0.40 μg/mg Cr, respectively. In urine samples collected from parturition to the next ovulation, 70.9% of the PdG-values were below 0.20 μg/mg Cr, whereas 99.2% of the urinary PdG concentrations measured during pregnancy were greater than this “threshold concentration”. A conception cycle was therefore defined as one in which the concentration of urinary PdG remained above 0.20 μg/mg Cr in all urine samples collected between day 1 and day 20 after ovulation. Gestation length was 151.5 ± 1.6 days (mean ± SEM, n = 6; range 147-157 days). The postpartum ovulation occurred 22.6 ± 4.7 days (mean ± SEM, n = 9; range 11-53 days) following birth. With the exception of two non-conception postpartum cycles observed in one female, with inter-ovulatory intervals of 26 and 27 days, postpartum ovulation resulted in conception, giving a 77.8% conception rate for nine observed cycles. The simple and rapid radioimmunoassay used in this study requires 5 h from urine collection to the final result, hence permitting daily monitoring of a large sample of females. It thus has important potential for conservation breeding programs and for other scientific investigations carried out with this endangered primate species. © 1994 Wiley-Liss, Inc.     

Effect of plane of protein after weaning on resumption of reproductive activity in Rasa Aragonesa ewes lambing in late spring

The effect of protein supplementation after weaning on the resumption of reproductive activity in a Spanish breed of ewes (Rasa Aragonesa) with a reduced seasonality was studied. Two equal groups of ewes were weaned in the first 2 weeks of July. From weaning to the end of the experiment both groups were fed identical energy-content rations but with different protein levels (high, Group H; low, Group L). The mean time between weaning and first detected estrus was 64 days. No differences between groups were found either in relation to this interval or in the ovulation rate in the first estrus. Ewes showing a multiple ovulation rate in the first cycle anticipated the onset of sexual activity after weaning. Significant differences in the ovulation rate were detected between the 2 groups from September to the end of the experiment (1.55 vs 1.05 corpora lutea for Groups H and L, respectively; P<0.001). In conclusion, the effect of a protein supplement after weaning in Rasa Aragonesa ewes lambing in late spring is reflected at mid-term in the early breeding season, with an evident rise of ovulation rate in the nutrient supplemented group.     

Changes in the nature of the estrous cycle and gametogenesis during superovulation stimulation in rats at different stages of the cycle

Estrous cycle, ovulation rate and gamete quality were studied in rats with superovulation induced on different days of the cycle. It was shown that the rat ovaries in estrous were most sensitive to the action of gonadotropins. Heterogeneity in the ovulation rate was noted in animals at the same phase of the cycle. There was a relative increase in the number of gametes with chromosomal alterations in experimental rats, with the value depending on the stage of the cycle when PMS was injected. The study of the estrous cycle in rats with superovulation induced on different days of the cycle has revealed strong hormonal influences on the sequence of stage alterations.     

The relationship between plasma progesterone and the timing of ovulation and early embryonic development in the marmoset monkey (Callithrix jacchus)

Levels of progesterone in peripheral plasma samples showed a mean (± S.E.M.) ovarian cycle length of 28 ·63 ± 1·01 days ( n = 19).
The preovulatory, or follicular phase (mean ± S.E.M. length: 8·25 ± 0·30 days, n = 56) was defined as that period of the cycle during which progesterone levels remained below 10 ng/ml. The postovulatory, or luteal phase (mean ± S.E.M. length: 19·22 ± 0·63 days, n = 48) was defined as the remaining period of the cycle during which levels remained between 10 ng/ml and 150 ng/ml.
The day of ovulation (day 0) was defined as that preceding the day on which progesterone levels first exceeded 10 ng/ml (day l), at the onset of the luteal phase. Oocytes and preim-plantation embryos recovered from the reproductive tract provided supporting evidence for the timing of ovulation.
The short follicular phase indicated that follicular growth may be initiated during the previous luteal phase. The long luteal phase may be related to the extended period of preim-plantation development.