BACKGROUND

Despite numerous advances in the field of in vitro fertilisation (IVF), many of the widely applied embryo culture techniques and resulting implantation rates have remained relatively unchanged since the first treatment was performed in the mid 1970's. Recent advances in the understanding of nutrient requirements of embryos, have led to a renaissance of extending their culture from the standard procedure of 2-3 days (early cleavage embryo transfer) to 5-6 days (blastocyst culture). The rationale for blastocyst culture is to improve the synchronicity of uterine and embryonic development and provide a mechanism for self-selection of viable embryos. Numerous reports on the clinical benefits of blastocyst culture have led to the worldwide introduction of this technique, despite a deficiency of conclusive evidence to do so.

OBJECTIVES

Primary: To determine if blastocyst stage embryo transfers (ET's) result in higher success rates, than cleavage stage embryo transfers. Secondary: To assess the overall embryo utilisation rate of both techniques.

SEARCH STRATEGY

Electronic searches of the Cochrane Menstrual Disorders and Subfertility Group specialised register of controlled trials, CCTR, MEDLINE, EMBASE, and Bio extracts were performed to identify relevant randomised controlled trials (RCTs). Attempts were also made to identify trials from the National Research Register, the Clinical Trial Register and the citation lists of review articles and included trials. The first or corresponding author of each included trial was also contacted for additional information.

SELECTION CRITERIA

Trials were included if they were randomised and compared the effectiveness of early cleavage versus blastocyst stage transfers.

DATA COLLECTION AND ANALYSIS

Of the 29 trials that were identified, ten trials met the inclusion criteria and were reviewed. Primary outcomes were rates of; live birth, clinical pregnancy and implantation per woman. Secondary outcomes were rates of; miscarriage, monozygotic twinning, embryo freezing, embryo utilisation, cancellation, multiple pregnancy and high order pregnancy and per cycle data. Quality assessment and data extraction were performed independently by two reviewers. Meta analysis was performed using odds ratios for dichotomous outcomes and weighted mean differences for continuous outcomes.

MAIN RESULTS

There was no significant difference between the two treatment groups in live birth rate, although this was reported by only one quasi-random trial (Peto OR 1.59, 95% CI 0.80, 3.15). There was also no evidence of a difference in pregnancy rate (both overall and subgroups) between the two groups for pregnancy rate per couple randomised (4 RCTs: Peto OR 0.86, 95% CI 0.57, 1.29). There was also no suggestion of an overall difference in implantation rates per embryo's transferred although it was impossible to calculate valid confidence intervals from published data (Day 2/3 17.1% vs Day 5/6 18.9%). The subgroup of sequential media trials suggested higher implantation rate for blastocyst transfer (Day 2/3 22.6% vs Day 5/6 32%). The miscarriage rate was no different between the two groups (1 RCT, Peto OR 1.66, 95% CI 0.41, 6.81). The RCTs reporting embryo freezing showed no difference (Peto OR 1.71, 95% CI 1.00, 2.94), however the two quasi-random trials showed a significant difference in favour of the Day 2/3 group (Peto OR 2.99, 95% CI 1.88, 4.75). Embryo transfer cancellation rates were significantly higher in the Day 5/6 group (5 RCTs: Peto OR 0.57, 95% CI 0.40, 0.83). There was no significant difference in the rate of multiple pregnancies or the rate of high order pregnancies (3 RCTs, Peto OR 0.58, 95% CI 0.30, 1.12)(2 RCTs, Peto OR 7.88, 95% CI 0.49, 126.30 respectively).

REVIEWER'S CONCLUSIONS: Overall this review of the best available evidence based on data from randomised controlled trials, suggests that to date little difference in the major outcome parameters has been demonstrated between early embryo transfer and blastocyst culture. Collectively, the increase in cancellation and the possible decrease in cryopreservation rates suggest that the routine practice of blastocyst culture should be offered to patients with caution. The subgroup of trials employing sequential media, did however demonstrate a substantial improvement in implantation rates and similar pregnancy rates, despite the transfer of less embryos. Whether this trend will culminate in convincing higher live birth rates per woman, has yet to be validated.