Implantation and pregnancy rates in relation to oestradiol and progesterone profiles in cycles with and without the use of gonadotrophin-releasing hormone agonist suppression

A drop in serum progesterone from oocyte pick-up +3 days to +5 days in fresh blastocyst transfer, using hCG-trigger and standard luteal support, is associated with lower ongoing pregnancy rates

STUDY QUESTION

Do early- and mid-luteal serum progesterone (P4) levels impact ongoing pregnancy rates (OPRs) in fresh blastocyst transfer cycles using standard luteal phase support (LPS)?

SUMMARY ANSWER

A drop in serum P4 level from oocyte pick-up (OPU) + 3 days to OPU + 5 days (negative ΔP4) is associated with a ∼2-fold decrease in OPRs.

WHAT IS KNOWN ALREADY

In fresh embryo transfer cycles, significant inter-individual variation occurs in serum P4 levels during the luteal phase, possibly due to differences in endogenous P4 production after hCG trigger and/or differences in bioavailability of exogenously administered progesterone (P) via different routes. Although exogenous P may alleviate this drop in serum P4 in fresh transfer cycles, there is a paucity of data exploring the possible impact on reproductive outcomes of a reduction in serum P4 levels.

STUDY DESIGN, SIZE, DURATION

Using a prospective cohort study design, following the initial enrollment of 558 consecutive patients, 340 fulfilled the inclusion and exclusion criteria and were included in the final analysis. The inclusion criteria were: (i) female age ≤40 years, (ii) BMI ≤35 kg/m2, (iii) retrieval of ≥3 oocytes irrespective of ovarian reserve, (iv) the use of a GnRH-agonist or GnRH-antagonist protocol with recombinant hCG triggering (6500 IU), (v) standard LPS and (vi) fresh blastocyst transfer. The exclusion criteria were: (i) triggering with GnRH-agonist or GnRH-agonist plus recombinant hCG (dual trigger), (ii) circulating P4 >1.5 ng/ml on the day of trigger and (iii) cleavage stage embryo transfer. Each patient was included only once. The primary outcome was ongoing pregnancy (OP), as defined by pregnancy ≥12 weeks of gestational age.

PARTICIPANTS/MATERIALS, SETTING, METHODS

A GnRH-agonist (n = 53) or GnRH-antagonist (n = 287) protocol was used for ovarian stimulation. Vaginal progesterone gel (Crinone, 90 mg, 8%, Merck) once daily was used for LPS. Serum P4 levels were measured in all patients on five occasions: on the day of ovulation trigger, the day of OPU, OPU + 3 days, OPU + 5 days and OPU + 14 days; timing of blood sampling was standardized to be 3-5 h after the morning administration of vaginal progesterone gel. The delta P4 (ΔP4) level was calculated by subtracting the P4 level on the OPU + 3 days from the P4 level on the OPU + 5 days, resulting in either a positive or negative ΔP4.

MAIN RESULTS AND THE ROLE OF CHANCE

The median P4 (min-max) on the day of triggering, day of OPU, OPU + 3 days, OPU + 5 days and OPU + 14 days were 0.83 ng/ml (0.18-1.42), 5.81 ng/ml (0.80-22.72), 80.00 ng/ml (22.91-161.05), 85.91 ng/ml (15.66-171.78) and 13.46 ng/ml (0.18-185.00), respectively. Serum P4 levels uniformly increased from the day of OPU to OPU + 3 days in all patients; however, from OPU + 3 days to OPU + 5 days, some patients had a decrease (negative ΔP4; n = 116; 34.1%), whereas others had an increase (positive ΔP4; n = 220; 64.7%), in circulating P4 levels. Although the median (min-max) P4 levels on the day of triggering, the day of OPU, and OPU + 3 days were comparable between the negative ΔP4 and positive ΔP4 groups, patients in the former group had significantly lower P4 levels on OPU + 5 days [69.67 ng/ml (15.66-150.02) versus 100.51 ng/ml (26.41-171.78); P < 0.001] and OPU + 14 days [8.28 ng/ml (0.28-157.00) versus 19.01 ng/ml (0.18-185.00), respectively; P < 0.001]. A drop in P4 level from OPU + 3 days to OPU + 5 days (negative ΔP4) was seen in approximately one-third of patients and was associated with a significantly lower OPR when compared with positive ΔP4 counterparts [33.6% versus 49.1%, odds ratio (OR); 0.53, 95% CI; 0.33-0.84; P = 0.008]; this decrease in OPR was due to lower initial pregnancy rates rather than increased overall pregnancy loss rates. For negative ΔP4 patients, the magnitude of ΔP4 was a significant predictor of OP (adjusted AUC = 0.65; 95% CI; 0.59-0.71), with an optimum threshold of -8.73 ng/ml, sensitivity and specificity were 48.7% and 79.2%, respectively. BMI (OR; 1.128, 95% CI; 1.064-1.197) was the only significant predictor of having a negative ΔP4; the higher the BMI, the higher the risk of having a negative ΔP4. Among positive ΔP4 patients, the magnitude of ΔP4 was a weak predictor of OP (AUC = 0.56, 95% CI; 0.48-0.64). Logistic regression analysis showed that blastocyst morphology (OR; 5.686, 95% CI; 1.433-22.565; P = 0.013) and ΔP4 (OR; 1.013, 95% CI; 0.1001-1.024; P = 0.031), but not the serum P4 level on OPU + 5 days, were the independent predictors of OP.

LIMITATIONS, REASONS FOR CAUTION

The physiological circadian pulsatile secretion of P4 during the mid-luteal phase is a limitation; however, blood sampling was standardized to reduce the impact of timing.

WIDER IMPLICATIONS OF THE FINDINGS

Two measurements (OPU + 3 days and OPU + 5 days) of serum P4 may identify those patients with a drop in P4 (approximately one-third of patients) associated with ∼2-fold lower OPRs. Rescuing these IVF cycles with additional P supplementation or adopting a blastocyst freeze-all policy should be tested in future randomized controlled trials.

STUDY FUNDING/COMPETING INTEREST(S)

None. S.C.E. declares receipt of unrestricted research grants from Merck and lecture fees from Merck and Med.E.A. P.H. has received unrestricted research grants from MSD and Merck, as well as honoraria for lectures from MSD, Merck, Gedeon-Richter, Theramex, and IBSA. H.Y. declares receipt of honorarium for lectures from Merck, IBSA and research grants from Merck and Ferring. The remaining authors declare that they have no conflict of interest.

TRIAL REGISTRATION NUMBER

The study was registered at clinical trials.gov (NCT04128436).

Serum progesterone level effects on the outcome of in vitro fertilization in patients with different ovarian response: an analysis of more than 10,000 cycles

OBJECTIVE

To investigate the relationship between serum P levels on the day of hCG administration and pregnancy outcomes in different responders undergoing IVF.

DESIGN

Retrospective study.

SETTING

Teaching hospital.

PATIENT(S)

A total of 11,055 women who underwent their first IVF/intracytoplasmic sperm injection cycles and a subgroup of 4,021 women undergoing frozen-embryo transfer (FET) cycles.

INTERVENTION(S)

Patients underwent IVF-ET with the long GnRH agonist protocol. The ovarian response was classified as high (≥ 20 oocytes; n = 2,023), poor (≤ 4 oocytes; n = 827), or intermediate (remaining cases; n = 8,205) according to the number of oocytes retrieved. Clinical outcomes of IVF-ET and FET cycles were analyzed according to plasma P levels.

MAIN OUTCOME MEASURE(S)

Ongoing pregnancy rates (PRs).

RESULT(S)

Ongoing PRs in fresh cycle were inversely associated with serum P levels on the day of hCG administration for all patients. Different P threshold concentrations were determined according to different ovarian response: We proposed a serum P level of 1.5 ng/mL as the threshold for poor responders, 1.75 ng/mL for intermediate responders, and 2.25 ng/mL for high responders. Our study does not show negative results for elevated P levels on oocyte performance in terms of fertilization, cleavage rate, or PR of FET cycles within different ovarian responses, offering no evidence for a detrimental effect of high P on oocyte quality.

CONCLUSION(S)

Elevated P levels on the day of hCG administration negatively influence PR regardless of different ovarian responses, although increased P threshold concentration is associated with better ovarian responses. The detrimental effect of P elevation on PR seems to be unrelated to oocyte quality in all responders.

Follicular dominance on the fifth day of controlled ovarian stimulation reduces implantation in long down-regulated ICSI cycles

OBJECTIVE

To determine if follicular dominance on the fifth day of controlled ovarian stimulation (COS) predicts implantation rates in down-regulated intracytoplasmic sperm injection (ICSI) cycles.

STUDY DESIGN

One hundred and sixty-two consecutive women undergoing ICSI treatment with long down-regulation and recombinant follicle-stimulating hormone injections were included in a prospective cohort analysis. The clinical pregnancy and implantation rates per transfer were compared between two groups, one with and the other without follicular dominance detected by ultrasound on the fifth day of COS.

RESULTS

There was no significant difference between the groups regarding the number of good quality embryos transferred, but clinical pregnancy and implantation rate per transfer were higher in group with follicular synchrony.

CONCLUSIONS

Follicular dominance on COS day 5 results in reduced implantation rates after ICSI.

Progesterone rise on the day of HCG administration (premature luteinization) in IVF: an overdue update

Premature luteinization (PL) refers to a rise in serum progesterone (P) levels on the day of hCG administration. Most studies used an absolute P level on the day of hCG administration as an indicator of PL, and the cutoff level differed from 0.8 to 2 ng/mL. Some authors defined PL as a P/E2 ratio of >1. There is a marked variation in the incidence (13% to 71%), of PL due to discrepancies in definition, population characteristics and/or treatment protocols. The pathogenesis of PL in COH is still poorly understood. Several hypotheses may be considered to explain this phenomenon: elevation of follicular LH levels, serum accumulation of HCG from HMG, increased LH receptor sensitivity of the granulosa cells to FSH, or poor ovarian response with increased LH sensitivity. The consequences of this premature elevation of serum P on IVF outcome remain controversial. Attempts to prevent COH include: use of Low-dose hCG alone in the late COH stages, flexible antagonist protocol, use of mifepristone, aspiration of a single leading follicle, hCG administration when the levels of serum P exceeded 1.0 ng/mL.

Is progesterone elevation on the day of human chorionic gonadotrophin administration associated with the probability of pregnancy in in vitro fertilization? A systematic review and meta-analysis

The role of progesterone elevation on in vitro fertilization (IVF) outcome has remained a debatable issue for several years. The aim of this systematic review and meta-analysis was to evaluate whether progesterone elevation on the day of human chorionic gonadotrophin (hCG) administration is associated with the probability of pregnancy. Eligible studies were considered those in which patients did not participate more than once. A literature search in MEDLINE, EMBASE and CENTRAL identified 12 eligible studies, 10 of which were retrospective. The majority (n = 10) of these studies did not detect a statistically significant association between progesterone elevation and the probability of pregnancy. Meta-analysis was performed only for the studies (n = 5) that provided data on clinical pregnancy per patient reaching hCG administration for final oocyte maturation. No statistically significant association between progesterone elevation and the probability of clinical pregnancy was detected (Odds ratio: 0.75, 95% confidence interval 0.53-1.06; P = 0.10). This finding persisted in the sensitivity analyses performed, which excluded the studies that did not report clearly that measurement of progesterone did not affect patients' management and those that did not report definition of clinical pregnancy. In addition, subgroup analyses were conducted on the basis of type of gonadotrophin-releasing hormone GnRH analogue used and on the value of serum threshold used to classify patients in those with or without progesterone elevation. These analyses, however, did not materially change the results obtained. In conclusion, the best available evidence does not support an association between progesterone elevation on the day of hCG administration and the probability of clinical pregnancy in women undergoing ovarian stimulation with GnRH analogues and gonadotrophins for IVF.

Progesterone supplementation to prevent recurrent miscarriage and to reduce implantation failure in assisted reproduction cycles

Implantation failure has been questioned for many cases of recurrent miscarriage and unsuccessful assisted reproduction. The exact cause of implantation failure is not known, but luteal phase defect is encountered in many of these cases. Consequently, women with recurrent miscarriages have been treated with progesterone supplementation with various degrees of success, and a recent meta-analysis has shown trends for improved live birth rates in those women. Progesterone probably acts as an immunological suppressant blocking T-helper (Th)1 activity and inducing release of Th2 cytokines. Numerous studies have confirmed that ovarian stimulation used in assisted reproduction is associated with luteal phase insufficiency, even when gonadotrophin-releasing hormone antagonists are used. In those patients, advanced endometrial histological maturity and a decrease in the concentration of cytoplasmic progesterone receptors are observed. Progesterone supplementation results in a trend towards improved ongoing and clinical pregnancy rates, except in patients treated with human menopausal gonadotrophin-only regimens, in whom ongoing pregnancy rates increase significantly. More randomized controlled trials are needed to increase the power of the currently available meta-analyses to further evaluate progesterone supplementation in both conditions.

The significance of premature luteinization in an oocyte-donation programme

BACKGROUND

Several evidences indicate that premature luteinization (PL) may affect IVF outcome. The primary end-point of the present study was to verify the effect of PL on the pregnancy rate (PR) of our oocyte-donation programme.

METHODS

PL was defined as serum progesterone > or = 1.2 ng/ml on the day of HCG. We analysed retrospectively 240 oocyte-donation cycles in which 120 women donated twice, with PL in the first donation cycle and no PL in the following one, acting as its own control. Recipients (n = 240) were divided in two groups according to the presence of PL (n = 120) or not (n = 120). Both groups were compared regarding donor cycle parameters and recipient cycle outcome.

RESULTS

There was no difference in PR between the groups (55.7 versus 54.4%, respectively). The number of total oocytes (18.2 +/- 0.6 versus 20.8 +/- 0.6; P = 0.003) and the number of mature oocytes retrieved (16.9 +/- 0.6 versus 19.4 +/- 0.6; P = 0.005) were different among donors with progesterone < 1.2 ng/ml and PL, respectively. There were no differences between the oocyte recipients in fertilization, cleavage, embryo division on day 3, blastocyst development or fragmentation rates. The number of embryos transferred, number of embryos cryopreserved, and implantation and miscarriage rates were similar between the groups.

CONCLUSION

PL does not appear to have a negative impact on ongoing PR in our oocyte-donation programme.

Predictive value of mid luteal progesterone concentration before luteal support in controlled ovarian hyperstimulation with intrauterine insemination

BACKGROUND

There is no published data assessing whether higher mid luteal serum progesterone (P4) levels are associated with a higher cycle pregnancy rate (CPR) in controlled ovarian hyperstimulation (COH) with intrauterine insemination (IUI).

AIMS

To assess whether the mid luteal serum P4 level is predictive of pregnancy in COH with IUI.

METHODS

A retrospective cohort study of all women with unexplained, minimal endometriosis or mild male factor infertility who underwent COH with IUI between October 1999 and December 2000 at our department was analysed. The COH was achieved with follicle stimulating hormone injections. All cycles were triggered with human chorionic gonadotropin when at least one follicle > or =15 mm was visible on ultrasound and IUI performed the following day. A serum P4 and beta human chorionic gonadotropin level was measured at 7 and 14 days post-trigger, respectively.

RESULTS

There were 33 pregnancies in the 188 cycles analysed, giving a CPR of 18%. The median (range) mid luteal P4 level for all cycles was 51 nmol/L (1.8-234). This did not differ between the pregnant (55 nmol/L) and non-pregnant (50 nmol/L) cycles (P=0.282, Mann-Whitney U-test). There was also no difference in CPR between cohorts below or above the cut-off levels of 33 nmol/L (25th percentile) (13.3 vs 18.9%; P=0.39), 51 nmol/L (50th percentile) (16.0 vs 19.1%; P=0.57), or 69 nmol/L (75th percentile) (16.3 vs 21.3%; P=0.44), respectively.

CONCLUSIONS

Increased mid luteal serum P4 levels are not associated with a higher CPR in women undergoing COH with IUI. However, a low mid luteal P4 level < or =25 nmol/L may help predict treatment failure.

Evaluation of different ovarian stimulation protocols for in vitro fertilization

In this study we evaluated retrospectively the efficacy of five different ovarian stimulation protocols in an in vitro fertilization program, in which 512 women were involved. Ovulation was induced by the following protocols: group I (271 cycles): buserelin short protocol (1 mg/day, intranasally) with human menopausal gonadotropin/human chorionic gonadotropin (hMG/hCG); group II (45 cycles): buserelin (short protocol) with pure follicle stimulating hormone (p-FSH)/hMG/hCG; group III (24 cycles): clomiphene citrate (100 mg/day) with hMG/hCG; group IV (122 cycles): hMG (3 ampules/day) and hCG; group V (113 cycles): hMG/hCG and prednisolone (7.5 mg/day) after cycle programming with oral contraceptives. The lowest cancellation rate (3.3%) was noted in group I, followed by group V (9.7%). The highest number of follicles was observed in groups I (8.3 +/- 0.3; mean +/- SEM) and V (7.8 +/- 0.5). Also, more oocytes were retrieved in group I (7.2 +/- 0.3, p < 0.001), which were of good quality based on oocyte maturity as well as on the fertilization rate, and more embryos (4.5 +/- 0.3, p < 0.05) were developed. The correlation between estradiol and the total follicular volume on the day of hCG administration was also examined in the five groups. The best correlation (r = 0.6502) was found in group I, followed by group V (r = 0.5810). Significant differences were observed in the five groups with regard to the number of hMG ampules administered (p < 0.0001, F = 15.393) and the stimulation days (p < 0.0001, F = 35.32). Sixty-six clinical pregnancies were achieved: 37 (17.5%) in group I, seven (25.9%) in group II, one (10%) in group III, ten (15.6%) in group IV and 11 (15.5%) in group V (differences were not statistically significant). In conclusion, all five protocols were satisfactory in ovarian stimulation for in vitro fertilization, and gonadotropin releasing hormone (GnRH) analogs seemed to be more advantageous by reducing the cancellation rate, enhancing the number of oocytes retrieved and embryos developed and by improving the pregnancy rates.

Selective use of leuprolide acetate in women undergoing superovulation with intrauterine insemination results in significant improvement in pregnancy outcome

OBJECTIVE

To examine the incidence of premature luteinization in individuals undergoing hMG with IUI therapy, the association between premature luteinization, cycle fecundity, and pregnancy outcome, and to determine if the selective use of leuprolide acetate (LA) in women demonstrating premature luteinization improves pregnancy outcome in subsequent hMG with IUI cycles.

DESIGN AND SETTING

Retrospective analysis of superovulation cycles from January 1990 until December 1991 at the University of Connecticut Health Center.

PATIENTS

All women with ovulatory function undergoing hMG superovulation with IUI.

INTERVENTIONS

All patients were tested for evidence of premature luteinization. Those demonstrating premature luteinization were started on LA in the luteal phase in their subsequent hMG with IUI cycle.

MAIN OUTCOME MEASURES

Peak serum E2, the number of mature preovulatory follicles, the number of ampules of hMG, days of hMG therapy, cycle fecundity, and spontaneous abortion rate.

RESULTS

Thirty-three percent of all hMG with IUI patients showed evidence of premature luteinization, with premature luteinization occurring in 22.2% of conception cycles and 37.4% of nonconception cycles. For those women who demonstrated premature luteinization in their conception cycle, 90.0% of the pregnancies ended with either spontaneous abortion or were biochemical in nature compared with 44.3% in the cycles without evidence of premature luteinization. Cycle fecundity was 11.1% in patients demonstrating premature luteinization compared with 26.3% for patients without premature luteinization. All women demonstrating premature luteinization and not conceiving were placed on LA in the luteal phase and had a subsequent cycle fecundity of 18.9% with the percent pregnancy wastage being significantly less (33.3% versus 90.0%) when LA was used.

CONCLUSIONS

Premature luteinization is a common occurrence during hMG therapy and is associated with decreased cycle fecundity and an increased incidence of spontaneous abortion and biochemical pregnancies. The selective use of LA in those individuals demonstrating premature luteinization results in a significant increase in the percent of women conceiving a viable pregnancy.

Hormone release from cultured luteinized-granulosa cells mimics differences seen in vivo in patients undergoing IVF-ET

OBJECTIVES

Previous research from this laboratory has suggested that a relationship exists between the increase in circulating progesterone concentrations at the time of hCG administration and cycle outcome in patients undergoing IVF. Progesterone (P) increases of threefold or better within the 24-h period surrounding hCG administration appeared to be associated with a higher pregnancy rate. These data suggest a functional difference in the luteinized-granulosa of patients undergoing IVF. To test this hypothesis:

DESIGN

A split-split plot arrangement of treatments with two cell sources under five hormonal stimulations at four time points.

METHODS

Luteinized-granulosa cells (LGC) were collected from patients with either a normal increase (> or = threefold = NC) in circulating P (n = 4) or those with lower P increases (AC; n = 4). The cells were isolated by Ficoll gradient centrifugation and then cultured in 24-well culture plates using a modified media 199 containing 100 mIU/ml of hMG, FSH, LH, hCG, or a nonhormonal control to stimulate steroid-hormone production. At time points 2, 4, 6, and 8 days, media from each well was frozen for later hormone assay and replaced with fresh media containing the same stimulating factor. After all the media had been collected, P and estradiol (E2) released into the media were measured using radioimmunoassay.

RESULTS

Results indicate a higher media concentration of P (P < 0.001), but not E2 (P = 0.254), from NC, regardless of hormone stimulation or time in culture, when compared to the media from AC. Media concentrations of E2 were affected by a cell source by hormone stimulation by time interaction (P < 0.001) with varying effects. Media from NC maintained a constant E2 of between 1000-3000 pg/ml over the 8-day period (P = 0.163). However, media from AC demonstrated a stimuli-dependent E2 release (P < 0.001) ranging from < 1000 to over 11,000 pg/ml.

CONCLUSIONS

These data appear to support the existence of functionally different populations of luteinized-granulosa cells from patients undergoing IVF-ET.

Serum progesterone concentrations on the day after human chorionic gonadotropin administration and progesterone/oocyte ratios predict in vitro fertilization/embryo transfer outcome

PURPOSE

In gonadotropin-releasing hormone analogue-pretreated in vitro fertilization-embryo transfer cycles, pregnancy rates are inversely related to serum progesterone levels on the day of administration of human chorionic gonadotropin. The relationship of the progesterone concentration on other days in the periovulatory period to pregnancy rates in such cycles is little studied. We therefore retrospectively analyzed the relationship between progesterone concentrations on the day after human chorionic gonadotropin and pregnancy in 114 cycles, 28 and 23 of which produced clinical and ongoing/delivered pregnancies, respectively. To assess the effect of the extent of follicular luteinization on success, we also studied the relationship between the progesterone concentration per oocyte retrieved and pregnancy for the day of and day after human chorionic gonadotropin.

RESULTS

Progesterone concentrations on the day after human chorionic gonadotropin were inversely associated with clinical pregnancy by multiple logistic regression analysis (P < 0.05). Progesterone/oocyte ratios were inversely associated with clinical pregnancy (P < 0.05) and ongoing/delivered pregnancy (P < 0.02) for both the day of and the day after human chorionic gonadotropin.

CONCLUSION

The study results extend the window of time during which elevated progesterone concentration is associated with poor outcome to at least 2 days. This finding is consistent with hypothetical mechanisms attributing the link between progesterone concentration and outcome to either endometrial or follicle/oocyte events. The association of lack of follicular luteinization (low progesterone per oocyte ratios) and favorable outcome suggests a predominant effect of progesterone on follicle/oocyte quality. Further studies are needed to clarify the mechanisms underlying the association between progesterone and in vitro fertilization-embryo transfer outcome.

The use of a liquid-phase controlled rate freezing system for the successful cryopreservation of mouse pre-embryos

OBJECTIVES

To investigate the ability of a liquid-phase, controlled-rate freezing machine to generate reproducible freezing gradients with a constant rate of change; temperature fluctuations and heat dissipation during seeding; to assess the viability of mouse pre-embryos exposed to the silicone liquid cooling phase and the rates of survival and viability of mouse pre-embryos cryopreserved using this system.

DESIGN

Freezing gradients were generated from the bath or a sample and compared for reproducibility and slope. Temperature fluctuations and gradients around the freezing chamber and the temperature rises and dissipation during seeding were measured. The toxicity of the silicone polymer freezing-phase was tested with mouse pronuclear pre-embryos. Different developmental stages of mouse pre-embryos were frozen and thawed and survival recorded in vitro and in vivo.

SETTING

Research Laboratories, Sinai Hospital of Baltimore, Baltimore, Maryland.

MAIN OUTCOME MEASURES

The reproducibility of freezing gradients; temperature fluctuations during seeding; and the in vitro and in vivo viability of mouse pre-embryos exposed to the silicone polymer or frozen and thawed.

RESULTS

The freezing gradients generated were constant and reproducible with a constant rate of change; no temperature differences were recorded around the freezing chamber; temperature changes at seeding are minimal and rapidly dispersed. The silicone polymer was nontoxic to mouse pre-embryos and mouse pre-embryos frozen with the system and subsequently thawed are viable both in vitro and in vivo.

CONCLUSIONS

This liquid-phase cryopreservation system is an attractive option for assisted reproductive technologies laboratories because liquid nitrogen is not required for operation, it is reliable, there are small temperature fluctuations during seeding, and it can be kept constantly running.