Results
483 stimulation cycles were undertaken in 373 women. The median age (SD) at freeze was 38.3 (±3.5). The median number of cycles per woman was 1 (range 1-8). The median number of oocytes retrieved per cycle was 8 (0-37), whilst the number of metaphase II oocytes was 6 (0-28). The median BMI was 22.5 (17.1-37) kg/m. The median baseline AMH was 9.9 (0.17-56.6) pmol/l and the AFC was 12 (2-49). At ovulation trigger, the median E2 level was 7.01 (0.25-30.26) per 1000 pmol/l and the total number of follicles ≥12mm was 9 (1-50). (Table 1)
Univariable Poisson regression was used to assess the effect of each variable on oocyte yield (Table 2) . This identified that age, baseline AFC, AMH, E2, and follicle count ≥12mm on the day of trigger were all significant predictors of oocyte yield (p<0.001). Multivariable Poisson regression subsequently demonstrated that BMI (Incidence rate ratio; IRR; 1.02; 95% CI 1.00 -1.03; 95% p=0.04), E2 level per 1000 pmol/l increase (IRR 1.05; 95% CI 1.04 -1.07; p<0.001) and the number of follicles ≥12mm at trigger (IRR 1.02; 95% CI 1.01-1.03; p<0.001) were all significant predictors of oocyte yield (Table 3) . However, there was no significant relationship between AFC (IRR=1.00; 95% CI 0.99-1.02 p=0.43) or AMH (IRR 1.00; 95% CI 0.99-1.01; p=0.71) with oocyte yield. An increase in age by one-year resulted in a 4% reduction in yield after adjusting for all other variables (IRR 0.96; 95% CI 0.94-0.98; p<0.001).
There were nine cases in total (2.4%) of OHSS; all of which classified as mild in severity as per the Royal college of Obstetrician and Gynaecologist classification.16 In regards to mode of trigger, there were four cases of hCG, four Suprefact and one case of dual trigger. There were no associated hospital admissions or significant adverse events. Analysis using the Mann Whitney U test identified that age, total AFC, number of follicles more than 12mm on the day of trigger and FSH were all significant predictors of OHSS (p<0.05). BMI, AMH and E2 levels however were not significant predictors of OHSS (Table 4).
Thirty-six women subsequently underwent 41 oocyte thaw cycles. The mean age at thaw was 42 (±4.1). The mean number of years between cryopreservation and thaw was 3.7 years. Almost two thirds (n=23) of women used partners sperm, whereas 36% (n=13) used donor sperm. Of those who used donor sperm, the majority did so because they were single (n=9, 69%).
The percentage of oocytes which survived thaw was 81, 76%, 67.5% in women aged ≤35, 36-39 and ≥40 years respectively. The fertilization rate of the frozen thawed oocyte was 53%, 68% and 58% in the respective age groups. There were 37 embryo transfers (ET). Almost half of the embryo transfers took place at the blastocyst stage 48.6%, (n=18). Of those cycles with all embryos cultured to the blastocyst stage, the average number of blastocysts formed were 55.6% (10/18), 45.5% (40/88) and 50% (9/18) in the respective age groups.
Following embryo transfer, 13 women had confirmed clinical pregnancies resulting in a clinical pregnancy rate (fetal heart visualised on ultrasound) of 31.7% per thaw cycle. 12 livebirths were subsequently achieved by 11 women (one twins), attributing a livebirth rate of 26.8% (11/41) per cycle and 29.7% per ET (11/37). The remaining two women had missed miscarriages, having both first cryopreserved their oocytes above 40 years old.
In women aged ≤35 years, two livebirths from six egg thaw cycles were achieved, resulting in a livebirth rate per egg thaw cycle of 33%. In those aged 36-39, 24 cycles led to 9 livebirths resulting in a livebirth rate of 37.5% per egg thaw cycle. In those aged 40 years and over, all 11 cycles were unsuccessful, resulting in a livebirth rate of 0% as demonstrated in Figure 1. The average age at cryopreservation of those who achieved a livebirth was 36.4, whereas those who did not was 39.0 (p=0.07).