Pregnancies resulting from in vitro fertilization using frozen embryos are associated with a higher risk of hypertension or complications related to hypertensive disorders than when fresh embryos are used or when the pregnancy occurs naturally.
That’s according to a study published Monday in the American Heart Association’s journal Hypertension, which included data on more than 4.5 million pregnancies over nearly three decades in three European nations: Denmark, Norway and Sweden.
The risk of pregnancy complications related to high blood pressure was higher after frozen embryo transfers compared with naturally occurring pregnancies, and the risk after new embryo transfers was similar to naturally occurring pregnancies, the data showed.
Further research is needed to determine whether similar findings will emerge in the United States.
The researchers – from the Norwegian University of Science and Technology and other European institutions – used Danish medical birth records dated between 1994 and 2014, Norway from 1984 to 2015 and Sweden from 1985 to 2015. Of the 4.4 million pregnancies conceived naturally, 78,300 pregnancies used fresh embryo transfer and 18,037 pregnancies from frozen embryo transfer.
The researchers compared the odds of hypertensive disorders during pregnancy in the groups and found that the unadjusted risk for these disorders was 7.4% after frozen embryo transfer, 5.9% after fresh embryo transfer and 4.3% after spontaneous conception. The data also showed that pregnancies resulting from the transfer of frozen and fresh embryos were more likely to be premature (6.6% of frozen and 8.1% of fresh, respectively) compared to naturally occurring pregnancies at 5%.
“Frozen embryo transfers are becoming more common worldwide, and in recent years, some doctors have begun to skip fresh embryo transfers in their clinical practice to freeze all embryos, a so-called ‘freeze all’ approach.” Dr. Sindre H. Petersen, lead author of the study. intern at the Norwegian University of Science and Technology in Trondheim, Norway, said in a news release on Monday.
“In summary, although most IVF pregnancies are healthy and uncomplicated,” he said, “this analysis found that the risk of gestational hypertension was significantly higher after frozen embryo transfer compared to fresh embryo transfer or natural conception pregnancies.”
Petersen added, “Our results highlight that the potential benefits and risks must be carefully considered before freezing all embryos as a routine in clinical practice.”
The findings are “consistent with previous population-level studies” showing an increased risk of hypertensive disorders in pregnancy after frozen embryo transfer, the researchers wrote in the study.
Last year, a large study presented online at the annual meeting of the European Society of Human Reproduction and Embryology also found an increased risk of preeclampsia and hypertension in pregnancies derived from frozen and thawed embryos, and the risk was found. be larger when the uterus was prepared for the introduction of hormone replacement therapies.
“The link between frozen embryo cycles in pregnancy and hypertensive disease has been known for a long time, and today there is an active debate about the pros and cons of ‘Freeze all for all?’ among fertility doctors,” said Dr. Ying Cheong, professor of reproductive medicine at the University of Southampton, in a statement released by the UK’s Science Media Center in July. He was not involved in either study.
“There are two important points to drive home here, firstly, although frozen embryo transfer technology has transformed reproductive medicine, FET should only be performed where it is clinically appropriate and secondly, clinicians and scientists need to understand what happens in early development and what happens in early development. they need to start connecting the dots between later birth and beyond, an area of research that I think is still poorly supported and understudied,” Cheong said.
The new study did not assess what might account for this association between frozen embryo transfer and high blood pressure risks, but some IVF doctors question whether it is really fresh or frozen.
“There is one thing that is not clear: from the actual embryo freezing procedure or from the protocol used? Most IVF doctors believe that based on recent research and evidence it’s actually the drug protocol, not the IVF procedure,” Dr. Aimee Eyvazzadeh, a reproductive endocrinologist in San Francisco who was not involved in the new study, wrote in an email to CNN on Monday.
“There are different ways to prepare for a uterus transfer,” she said. One protocol involves a cyst of the corpus luteum, a fluid-filled mass that forms in the ovaries and plays an important role in pregnancy, as the corpus luteum produces the hormone progesterone needed during pregnancy. Another protocol relies on medication to mimic ovulation.
“Research shows that it is the lack of a corpus luteum that increases the risk and therefore a frozen transfer may increase the risk of preeclampsia,” Eyvazzadeh wrote.
Overall, the new study is “extremely important” to “anyone who cares for people who become pregnant after IVF,” she wrote. “Everyone caring for people who are pregnant after IVF should pay close attention to this study. More and more studies show what IVF doctors already know, which is that IVF after frozen embryo transfer can increase the risk of preeclampsia.”