Pre-eclampsia and high blood pressure in pregnancy may be linked to the way the uterus is prepared before frozen embryos are transferred.
Research presented at the meeting of the European Society of Human Reproduction and Embryology 2021 showed that pregnancies, where the uterus is primed with hormone therapy before a frozen embryo is transferred, have higher rates of hospitalisation from vascular problems.
Dr Sylvie Epelboin from the Hôpital Bichat-Claude Bernard in Paris, France presented the research which was carried out on behalf of the French Biomedicine Agency.
This was the first large-scale trial to look into the correlation between the method of endometrial preparation and high blood pressure in pregnancy, although other studies have reported that rates of pre-eclampsia are higher in pregnancies resulting from frozen embryo transfers compared to fresh embryo transfers or natural conception.
The study looked at records of almost 70,000 single-baby IVF pregnancies between 2013 and 2018, of which around 20,000 were from frozen embryo transfers. None of the pregnant women had a history of high blood pressure.
The researchers further stratified the patients into those who had received hormone stimulation to prepare the uterus for the embryo to implant (AC-FET: artificial cycle frozen embryo transfer) and those whose natural cycle was used (OC-FET: ovulatory cycle frozen embryo transfer). There were around 10,000 pregnancies in each group.
The study showed that risk of pre-eclampsia and other blood pressure related problems was very similar in the fresh embryo transfer and the OC-FET groups, but significantly higher in the AC-FET group.
According to Dr Epelboin, there are two possible reasons for this discrepancy: one is that the high and prolonged doses of artificial hormones administered in AC-FET could have a role in increasing the risk of high blood pressure. The other possibility she discussed is that the corpus luteum suggested that it could have a protective effect against vascular problems in pregnancy. The corpus luteum is a group of cells left behind after each ovulation that produce hormones that naturally help establish and maintain pregnancy, and would be present in natural conception, fresh embryo transfers and OC-FET.
Dr Epelboin recommended that OC-FET should be preferred as a first-line protocol where possible as there does not appear to be a difference in pregnancy rates in frozen embryo transfers using the natural ovulatory cycle or hormone replacement.