Fresh vs. frozen embryo transfer – what is the effect of ovarian response on live birth rates?

This study investigated if ovarian response (OR) has an effect on pregnancy outcomes after embryo transfer (ET).

They found that frozen ET (fzET) was associated with better pregnancy outcomes in women with high OR. 

Assisted reproduction (AR) is used to treat infertility. It involves a number of steps. Controlled ovarian stimulation (COS) is used to harvest eggs. One of the limits of AR is the ovarian response (OR). This term describes how many eggs are retrieved during COS. Once eggs are harvested they may undergo in vitro fertilization (IVF) or intracytoplasmic sperm injection (ICSI). After IVF/ICSI embryos can be transferred directly to the uterus. They can also be frozen for transfer at a later stage.

Many studies have compared fresh (fr) and frozen (fz) embryo transfer (ET). FzET appears to be the most successful option. Most studies have not examined other factors that could affect pregnancy outcomes after ET. OR is one of these factors. High OR can be a sign of ovarian hyperstimulation syndrome (OHSS). OHSS can have negative effects for the mother and pregnancy. It is unclear if OR has an effect on pregnancy outcomes according to the type of ET. 

This study included 2514 ET cycles. Patients underwent frET or fzET at their discretion. COS and IVF/ICSI were performed using a standard protocol. For the analysis patients were grouped according to their OR. The main pregnancy outcome evaluated was the cumulative live birth rate (CLBR). The rate of OHSS also investigated. 

CLBR was similar between frET and fzET in women with an OR of 10 or less (OR 10-). In women with an OR of 1-5, CLBR was 46.1% for frET and 43.3% for fzET. In women with an OR of 6-10, CLBR was 60% for frET and 66.7%. 

Women that had an OR of more than 10 eggs (OR 10+) had higher CLBR with fzET. In women with an OR of 11-15, CLBR was 71.3% for fzET and 57.8% for frET. In women with an OR of 16-25, CLBR was 71.5% for fzET and 65.2% for frET.

OHSS rates were also higher in the OR 10+ group after frET. In the 11-15 OR group OHSS rates were 3.4% with frET and 0% with fzET. In the 16-25 OR group OHSS rates were 10.9% for frET vs 0.5% for fzET. Birth weights of newborns were significantly lower in frET pregnancies. 

The authors concluded that fzET was associated with better pregnancy outomes in women with high OR.

This study was based on medical records. Not all information was available. More patients in the OR 10+ groups underwent fzET. A controlled study is needed to confirm the results.