Single-embryo transfer has been proposed as a way of limiting multiple births after in vitro fertilization (IVF) in women less than 36 years of age. Extending embryo culture from the 3-day cleavage stage to the 5-day blastocyst stage might make pregnancy likelier and, in addition, the morphologic criteria that guide embryo selection are less subjective at the later date. This study analyzed the outcome of IVF in 351 infertile women age 35 and younger; 176 were assigned to have transfer of a single cleavage-stage embryo, whereas 175 received a single blastocyst-stage embryo. Multifollicular ovarian stimulation was carried out using recombinant follicle-stimulating hormone and a gonadotropin-releasing hormone antagonist. Oocyte maturation was promoted by administering human chorionic gonadotropin, and the luteal phase was supported by vaginally administered progesterone.
More than 90% of women in both treatment groups were having their first or second trial of IVF or intracytoplasmic sperm injection. The groups were similar demographically and in embryologic characteristics, but significantly more embryos in the cleavage-stage group were cryopreserved. Approximately two thirds of each group had a top-quality embryo for transfer. Compared with cleavage-stage cases, the blastocyst-stage group had a significantly higher rate of ongoing pregnancy (33% vs 22%). The relative risk (RR) was 1.54 with a 95% confidence interval (CI) of 1.08 to 2.18. Deliveries also were significantly more frequent in the blastocyst-stage group (32% vs 22%); the RR was 1.48 with a 95% CI of 1.04 to 2.11. A higher rate of first-trimester pregnancy loss in the cleavage-stage group (34% vs 19%) may be partly responsible, although the difference was not statistically significant. Two of 94 deliveries (2%), both in the cleavage-stage group, resulted in monozygotic twin births.
These results affirm the effectiveness of single-embryo transfer in reducing the risk of multiple births. In younger women having their first or second trial of IVF, transfer of a single blastocyst-stage embryo is significantly likelier than use of a cleavage-stage embryo to result in pregnancy and delivery.