Many questions are asked about the freezing of embryos and its effect on pregnancy rates. How long do frozen embryos stay viable? Is there a lesser chance of pregnancy down the road? What effect will freezing embryos have on the fetus? Numerous studies have been performed that prove embryo freezing is safe and effective. The embryos are viable for long periods of time and have produced many successful pregnancies.

In fact, a recent study reported at the European Society for Human Reproduction and Embryology discovered that children born from frozen embryos do better and have a higher birth weight than children born from a fresh transfer.

Children from frozen embryos study

The study was conducted out of Copenhagen and evaluated babies born during the years 1995-2006. 1267 children born after Frozen Embryo Replacement (FER) were studied and categorized into three groups. 878 of them were born using frozen embryos that were created using standard in vitro fertilization in which the sperm are placed into a dish close to the egg but must penetrate the egg on their own. 310 children born with frozen embryos created using ICSI in which a single sperm is injected into a single egg, and 79 were born where the method of creation of the embryos was unknown .

17,857 babies born after a normal IVF/ICSI with fresh embryos were also studied and used as a control group or reference group. Data on all of the children’s outcomes was taken regarding birth defects, birth weights, and length of pregnancy.


The results of the Copenhagen study showed that the children who came from frozen embryos had higher birth weights, longer pregnancies, a less pre-term births. There was no difference in the rate of birth defects whether the children came from frozen embryos or fresh embryos. In the FER group, the birth defect rate was 7.7% compared to the fresh transfer group which was slightly higher at 8.8%. The scientists also found that the risk for multiple pregnancies was increased in the fresh embryo transfers. Around 11.7% of the ICSI and 14.2% of the IVF frozen cases were multiple pregnancies. In the case of fresh embryos, 24.8% of the ICSI and 27.3% of the IVF were multiple pregnancies. It should also be noted that maternal age was significantly higher in the FER group. This is significant since based on age one would have expected a higher rate of problems and birth defects.


This study adds to the body of knowledge suggesting that embryo freezing is a safe procedure. It is unclear why the frozen embryo children did better than their fresh embryo counterparts. One suggestion is that "weaker embryos" may have been weeded out by the freeze thaw process so that only the healthiest embryos survived to produce a pregnancy. This seems unlikely however since there is much variation between programs as to the type of embryos they will freeze and even the stage of development that freezing is performed at.

Another possible explanation has to do with the use of fertility medications during IVF cycles. during these cycles,  hormone levels are driven much higher than they are naturally. In frozen embryo replacement cycles, hormone supplementation is usuallly given but at levels that are much closer to those seen naturally.

Future studies will need to look at these factors as well as other questions such as whether there are differences in the children depending on the length of time the embryos were frozen.