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In the wake of the Alabama Supreme Court decision establishing that frozen embryos can be considered children, some IVF clinics in that state have paused operations -- because discarding embryos is inherent to the process.

There are a few reasons why embryos are discarded, reproductive endocrinologists told MedPage Today: they stop growing and won't lead to a pregnancy; genetic testing reveals chromosomal abnormalities; or some embryos are left over after patients are done building their families.

Additionally, if IVF results in a multifetal pregnancy, professional guidance recommends reducing the number of fetuses to have a safer pregnancy.

Many patients will need multiple cycles of IVF to have a successful pregnancy, said Rachel Weinerman, MD, an ob/gyn and reproductive endocrinologist at Case Western Reserve University in Cleveland. Thus, there are many instances where embryos must be discarded.

In IVF, embryos are grown in culture for about 5 days to see which ones develop enough to be transferred; only about half make it to that blastocyst stage, on average, reproductive endocrinologists said.

Spencer Richlin, MD, the surgical director and partner at Illume Fertility, a Connecticut-based fertility clinic, told MedPage Today that "the tricky part is that not every egg is going to make an embryo," and some people require more eggs and embryos to have a chance at pregnancy.

The rate of how many eggs will become embryos is different for every person, but it's usually dictated by age, he explained. Weinerman added that for most women under 35, most of the eggs will be normal. But for older women, most eggs will be abnormal.

Both Weinerman and Richlin noted that human reproduction is not particularly efficient. And for IVF, it's not as simple as one egg resulting in one ideal embryo that results in a successful pregnancy, and thus, a baby. A lot has to go right, and still, a lot comes down to chance. So whenever possible, doctors and patients look to increase the chance of success.

One of those strategies is pre-implantation genetic testing (PGT), which Richlin said is commonly performed on embryos to screen out ones that don't have 46 chromosomes and ones that contain unwanted genes -- like genetic conditions the parents don't want to pass down. He said about 60% of patients opt for PGT.

Even in the best-case scenarios -- a woman is young, the blastocyst embryos are genetically normal, and everything goes to plan -- the rate of getting pregnant is only about 60%, Weinerman said, hence why it often involves multiple attempts.

Embryos can be stored in liquid nitrogen freezers indefinitely. Weinerman said that a lot of practices freeze embryos after culture, so that patients can continue using the embryos until they finish family building.

Richlin said that 99% of the time, when people are done with family building and have no need for any remaining embryos, they opt to have them discarded. But he also noted that, in rare circumstances, his IVF patients have opted for "embryo donation" -- where a patient's embryo is donated to another person trying to build a family. However, this donation process involves legal permissions, social work, and additional discussions. In other situations, a person may have stored frozen embryos before they passed away. Those are often discarded, too.

When it comes to transferring embryos, the American Society for Reproductive Medicine (ASRM) recommends that only one is transferred at a time in most situations, according to .

For instance, in patients with favorable prognosis up to age 37, the guidance recommends a single embryo transfer -- and same goes for patients of any age with a preexisting condition that increases the risks associated with multifetal pregnancies. For those ages 38 to 40, no more than two blastocysts should be transferred. And for patients 41 or 42, the guidance recommends transferring no more than three blastocysts at a time.

Overall, it's rare to implant more than one embryo at a time, Richlin said, and the most instances of implanting more than one embryo is when a patient is over 38 and has opted out of PGT.

He also noted that an implanted embryo splits into a twin pregnancy about 1% of the time but there's a 50% chance of twins when two genetically tested blastocysts are implanted. In order to have a triplet pregnancy, at least two embryos would have to be implanted and one would have to split.

Richlin said his clinic sees maybe one triplet pregnancy per year. IVF patients who end up with triplet pregnancies are counseled on their options and most chose fetal reduction, where an ultrasound-guided injection into one or more of the fetuses stops the fetal heart.

ASRM guidelines note that while "multifetal pregnancy reduction can be performed to reduce the fetal number, the procedure may result in the loss of all fetuses, it does not completely eliminate the risks associated with multiple pregnancies, and it may have adverse psychological consequences."

The American College of Obstetricians and Gynecologists (ACOG) similarly that multifetal pregnancies should be avoided whenever possible and when they do occur, patients should be counseled on the additional risks and complications that multifetal pregnancies bring.

"Our job as infertility physicians is to help bring life into this world and to help give children to people who desperately want them," Weinerman said. "It's a shame if restrictive views of embryos would prevent these children from being born."

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