Objectives: To determine if the success of frozen embryos obtained from assisted reproductive technology (ART) cycles is dependent upon the outcome of the ART cycle from which they were derived and to determine if the length of time in cryostorage affects pregnancy rates (PRs).
Design: Retrospective analysis of pregnancy outcome of consecutive frozen ETs compared with their corresponding "'fresh" cycles.
Setting: University-affiliated private ART program.
Patients: Between July 1986 and December 1992, 375 oocyte retrieval cycles had at least one subsequent frozen ET for comparison. Of the 375 patients, 94 achieved a clinical pregnancy during their fresh cycle (group A) and 281 patients did not (group B).
Interventions: Frozen ETs were performed using either natural (unstimulated) cycles (n = 79) or artificial cycles (n = 296). Transfers during natural cycles were timed with a combination of serum LH levels and ultrasound (US). These transfers were performed 2 days after ovulation. Ovarian suppression with leuprolide acetate followed by sequential estrogen and P replacement were used in the artificial cycles. These transfers were performed on the 3rd day of P administration. Clinical pregnancies were defined as the presence of a gestational sac on transvaginal US.
Main outcome measures: Patients were compared based on the pregnancy outcome of their frozen ET cycle and fresh ART cycle. chi 2 analysis and Student's-tests were used to test for statistical significance.
Results: Twenty-three patients (24.1%) from group A achieved a pregnancy from their frozen transfers compared with only 24 patients (8.5%) in group B who achieved a pregnancy from their frozen cycle. PRs did not differ based on the method of follicle aspiration, laparoscopy versus transvaginal US retrieval. Significantly lower PRs were noted in frozen ETs done within the first 10 months after cryopreservation compared with fresh cycle PRs.
Conclusions: Sibling embryos from a prior successful ART cycle are more likely to initiate a frozen pregnancy and prolonged cryostorage did not affect PRs. This higher PR probably reflects better quality in both fresh and frozen embryos. Therefore, the outcome of the initial cycle can be used to predict the success or failure of subsequent frozen transfers and oocyte-embryo quality appears to be key.