Frozen embryo transfers linked to high blood pressure risks during pregnancy

Pregnancies from fresh embryo transfers - transferring the fertilised egg immediately after in vitro fertilisation (IVF) rather than a frozen, fertilised egg - and natural conception had a similar risk of developing a hypertensive disorder, according to the study.

High blood pressure during pregnancy frequently indicates preeclampsia, a pregnancy complication characterised by persistently high blood pressure that can endanger the mother's and fetus's health and life. According to the American Heart Association, preeclampsia affects about one out of every 25 pregnancies in the United States.

Frozen embryos are used in one IVF treatment method: after an egg is fertilised by sperm in the lab, it is frozen using a cryopreservation process before being thawed and transferred to the uterus at a later date. According to the study authors, the procedure is becoming more common due to significantly improved freezing technology or cryopreservation methods that began in the late 2000s and because more patients are choosing to freeze embryos. However, frozen embryo transfer is linked to a higher risk of hypertensive disorders in pregnancy than both natural conception and fresh embryo transfer. However, it was unknown prior to this study whether this was due to the freezing process or a risk factor from the parents.

"Frozen embryo transfers are now increasingly common all over the world, and in the last few years, some doctors have begun skipping fresh embryo transfer to routinely freeze all embryos in their clinical practice, the so-called 'freeze-all' approach," said Sindre H. Petersen, M.D., the study's lead author and a PhD fellow at the Norwegian University of Science and Technology in Trondheim, Norway.

Researchers examined national data from medical birth registries in Denmark, Norway, and Sweden on nearly 2.4 million women aged 20 to 44 who had single deliveries and gave birth between 1988 and 2015. These data served as the foundation for a population-based study that also included a sibling comparison of women who had both an IVF pregnancy and a naturally conceived pregnancy. This method was used to determine whether the potential cause of the hypertensive disorders was parental factors or the IVF treatment.

The study included over 4.5 million pregnancies, of which 4.4 million were naturally conceived; over 78,000 were fresh embryo transfers, and over 18,000 were frozen embryo transfers. More than 33,000 pregnancies were grouped for sibling comparison among all pregnancies - mothers who conceived using more than one of these methods. The study is the largest sibling comparison study to date. The odds of developing hypertensive disorders in pregnancy after fresh vs. frozen embryo transfers were adjusted for factors such as birth year and mother's age.

"In summary," Petersen said, "most IVF pregnancies are healthy and uncomplicated." "This study discovered that the risk of high blood pressure during pregnancy was significantly higher after frozen embryo transfer compared to fresh embryo transfer or natural conception."

* In the population analysis, women whose pregnancy was the result of a frozen embryo transfer were 74% more likely to develop hypertensive disorders in pregnancy than those who conceived naturally.

* Among women who had both a natural conception and a frozen embryo transfer IVF conception (the sibling comparison), the risk of hypertensive disorders in pregnancy after frozen embryo transfer was twice as high as in pregnancies without frozen embryo transfer.

* Pregnancies resulting from fresh embryo transfer were not associated with a higher risk of developing hypertensive disorders when compared to natural conception, neither at the population level nor in sibling comparisons.

"Our sibling comparisons show that the increased risk is not due to factors related to the parents, but rather to some IVF treatment factors," Petersen said. "Future research should look into which aspects of the frozen embryo transfer process may influence the risk of hypertension during pregnancy."

Among other findings, women in the study who gave birth after IVF pregnancies were 34 years old on average for frozen embryo transfer, 33 years old for fresh embryo transfer, and 29 years old for natural conception. Preterm birth occurred in approximately 7% of babies conceived through frozen embryo transfer and 8% of babies conceived through fresh embryo transfer, compared to 5% of babies conceived naturally.

In addition to preeclampsia, the researchers defined gestational hypertension, eclampsia (the onset of seizures in those with preeclampsia), and chronic hypertension with superimposed preeclampsia as a combined outcome.

The lack of data on the type of frozen embryo cycle limited the study's ability to pinpoint which part of the frozen cycle or frozen transfer may contribute to the increased risk of hypertensive disorders. Another limitation is that data from Scandinavian countries may limit the findings' generalizability to people in other countries.

"Our findings highlight the importance of carefully weighing all benefits and potential risks before freezing all embryos as a routine in clinical practice." "A comprehensive, personalised discussion between physicians and patients about the benefits and risks of fresh vs. frozen embryo transfer is critical," Petersen said. (ANI)