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Stanford University School of Medicine

Waiting to transfer embryos results in more pregnancies for some IVF patients

Undergoing in vitro fertilization can be a long, stressful process for women who want to become pregnant. The outcome of each step is uncertain: Will the hormones used to mature a woman's eggs work? If they can harvest eggs successfully, will the medical team be able to create healthy embryos in the lab? Will those embryos, once transferred back to the woman's uterus, implant to establish a successful pregnancy?

New research published today in the journal Fertility and Sterility helps make one step in the process more certain.

After an IVF team creates embryos, the patient and her doctors must decide whether to try implanting an embryo a few days later (a "fresh" embryo transfer) or freeze all of the embryos and wait for a subsequent hormonal cycle to try to start a pregnancy. In the early days of IVF, fresh transfer was standard, but recent improvements in freezing and the growth of genetic tests that occur before embryo transfer have shifted the common practice. This shift made doctors wonder if freezing all embryos might actually lead to higher pregnancy rates. Was it worth advising patients to wait a little longer to try to become pregnant?

The new findings, from a team led by Stanford's Ange Wang, MD, a resident in obstetrics, show that in some cases, an extra bit of waiting between egg retrieval and embryo transfer is helpful. In an analysis of nearly 3,000 attempts to establish IVF pregnancies, Wang and her collaborators showed that women older than 35 and those with high levels of the reproductive hormone progesterone had higher pregnancy rates if all of their embryos were frozen to begin with.

"This finding is important because it may suggest a group of women that benefits more from freeze-all IVF cycles," Wang said in our press release.

The key, she explained, is that the extra time between egg harvest and the attempt to start pregnancy may allow a patient's too-high progesterone level to fall. "Higher progesterone levels may make it more difficult for embryos to implant — that is, adhere to the wall of the uterus to establish pregnancy — possibly due to premature maturation of the uterine lining," Wang told me.

Women's wishes should always be considered, Wang added: If someone wants to go ahead with a fresh embryo transfer, physicians should find out more about why they don't want to wait. But the new findings may also help ease the strain of a bit more waiting for women who are navigating the many uncertainties of IVF.

Previously: New process helps patients make decision on IVF embryo donation, Study shows mothers receiving fertility treatments may have an elevated risk of depression and Stanford IVF research on Time's top ten list
Image by Jenn Lewald

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