Pregnant people who have undergone frozen embryo transfers may be more likely to develop high blood pressure during pregnancy, a Norwegian study published this month found.
Researchers included more than 4.5 million pregnancies, including some that were naturally conceived and those done via fresh or frozen embryo transfers.
Participants were 20 to 44 years old and gave birth between 1988 and 2015.
Frozen embryo transfer pregnancies were 74% more likely to develop hypertensive disorders during pregnancy compared with those who conceived naturally. The findings were published in the peer-reviewed American Heart Association journal Hypertension.
What else did the researchers find?
The team also looked at factors among the parents, such as socioeconomic status, underlying lifestyle and health behaviors, and genetics, said Dr. Sindre H. Petersen, the study's lead author and a Ph.D. fellow in the department of public health and nursing at the Norwegian University of Science and Technology.
Some participants in the study gave birth to multiple children via both natural conception and frozen embryo transfer IVF conception. But even when the researchers examined parental factors among siblings, the risk for hypertensive disorders among frozen embryo transfer pregnancies was twice as high compared with pregnancies from natural conception, Petersen said.
"(Because) the findings persist in sibling comparisons, we are more certain that some aspect of the frozen embryo transfer technology is the reason, and not parental factors," Petersen told USA TODAY via email.
One of the study's biggest limitations, he said, is that the researchers don't know what part of the frozen embryo transfer process increases the risk for hypertensive disorders.
More research is needed to find that out, he said.
Dr. Sadiya Khan is an assistant professor of medicine and preventive medicine at Northwestern University's Feinberg School of Medicine. She wasn't involved in the study but said that typically, there's a reason people go the frozen embryo route, and that reason is quite different from someone who's able to naturally conceive. Those factors may be part of the reason for the increased risk in hypertensive disorders during pregnancy.
"For example, the age was different among people who had frozen embryo transfers compared with those who had conceived naturally," she said. "Even a three-year difference can also be associated with a difference in hypertensive disorders in pregnancy."
What does this mean for parents and their babies?
It's a serious pregnancy complication that could be life-threatening to mothers and their babies, the researchers said.
Khan, from Northwestern University, said medical professionals had thought these hypertensive disorders went away once the pregnancy ended, but some people can still have it during the first few weeks postpartum.
"It has been associated with long-term risk of cardiovascular disease," she said. "Even if after pregnancy, your blood pressure goes back to normal, there is a higher risk of the developing heart disease later in life. It's an important red flag for cardiovascular prevention across the lifetime."
The rates of hypertensive disorders during pregnancy have doubled in the past decade, she said. The frozen embryo study might help medical professionals identify risk factors for them; that way, they can pinpoint who's at risk and consider preventive therapies, monitoring or other interventions.
But the study's findings are not meant to scare those seeking IVF treatment, said Petersen. The majority of IVF-pregnancies (both fresh and frozen embryo transfers) are "perfectly healthy and uncomplicated."
The results are useful though, especially because there is an ongoing discussion in the IVF-community about freezing all embryos as a routine, or the “freeze all” approach, he said.
"A well-grounded and individualized decision of whether to go for a fresh or frozen embryo transfer can be made after dialogue between the clinicians and their patients."
Saleen Martin is a reporter on USA TODAY's NOW team. She is from Norfolk, Virginia – the 757 – and loves all things horror, witches, Christmas, and food. Follow her on Twitter at @Saleen_Martin or email her at firstname.lastname@example.org.
This article originally appeared on USA TODAY: High blood pressure more likely among frozen embryo pregnancies: study