Why Using Frozen Embryos To Become Pregnant Can Lead To More Complications

Using frozen embryos for in vitro fertilization (IVF) may increase the risk of hypertensive disorders during pregnancy, according to a recent study published in the journal Hypertension. In the study, researchers analyzed the medical data of millions of pregnancies in Norway, Denmark, and Sweden between 1988 and 2015. According to the American Heart Association News, researchers compared 4.4 million naturally conceived pregnancies to 78,000 pregnancies from fresh embryo transfers and 18,000 pregnancies from frozen embryo transfers.

Compared to naturally conceived pregnancies, those who used frozen embryo transfers during IVF treatments were 74% more likely to develop hypertensive disorders during pregnancy, like preeclampsia — a serious blood pressure condition that can endanger the health and life of the fetus and pregnant person. Those who used fresh embryos, however, did not see an increased risk of hypertensive disorders. This suggests that the higher risk of hypertensive disorders associated with the use of frozen embryos during IVF is not caused by parental risk factors but by the freezing process.

Having high blood pressure during pregnancy won't necessarily lead to further complications, but it's still important for doctors to monitor pregnant patients who develop hypertension. Of the pregnant people who developed hypertensive disorders in the study, 75% also developed preeclampsia and signs of liver or kidney damage by 20 weeks of gestation, which can be fatal if left untreated (via NBC News). "If our study only found that there was an association just for isolated gestational hypertensive disorders, we wouldn't be that worried, but we found in our extra analysis that there was a strong association between frozen IVF and preeclampsia," Dr. Sindre Petersen, a doctoral fellow at the Norwegian University of Science and Technology and the study's lead author, told NBC News.

Although it's still unclear exactly why using frozen embryo transfers can increase the risk of hypertensive disorders, one possible explanation is the absence of the corpus luteum — a temporary organ that develops on the follicle of an ovary during ovulation, which secretes early pregnancy hormones, like progesterone and relaxin, that thicken the uterine walls. This process is bypassed during frozen embryo transfers, however. While a previous study was able to make this connection, the sample size was overwhelmingly comprised of white participants.