Ever wonder how fertility doctors figure out which embryos to transfer during IVF? Here are the criteria they use to make this all-important decision.
Life as an in vitro fertilization (IVF) patient means you’re pretty much in a constant state of suspense: How well am I responding to the fertility drugs? How many follicles are there? How many eggs? How many of them fertilized? If you’ve gotten as far as the embryo transfer, you can add a new question: How good are my embryos? This is the time when the lab specialists at your fertility clinic evaluate your fledgling offspring to decide which one(s) should be plucked from the Petri dish and transferred into your uterus.
This is an all-important step, since an embryo’s report card determines to a large extent your chances of getting pregnant. In recent years, embryologists have become more adept at judging an embryo’s potential to implant, which is one reason that IVF success rates, particularly for younger patients, have risen from 15 percent in the 1980s to more than 50 percent today. And fertility scientists are developing even more sophisticated ways to evaluate embryos. “When we arm ourselves with these tools, we will see an increase in pregnancy rates because we’ll be using more information to guide us,” says Kathryn J. Go, Ph.D., scientific and laboratory director at the Reproductive Science Center of New England, a group of clinics in the Boston area.
IVF labs use a variety of embryo grading systems involving letters and/or numbers – a “Grade 3” or a “4AA,” for instance – but all of the scores are based on morphology, or what an embryologist can see through a microscope. The most important factor determining exactly how your embryos will be evaluated is when you’re scheduled for your embryo transfer – at day 3 or day 5 following fertilization. Embryos transferred on the third day (a.k.a. “day-3 embryos”) are graded according to different criteria from those transferred on day 5. Whether you have a day-3 transfer or a day-5 transfer depends largely on how many of your eggs fertilized, though embryo quality matters too. If you have plenty of embryos, your fertility doctor may wait until day 5 on the theory that the extra two days will weed out the weaker embryos. If you’re short on embryos, though, a day-3 transfer may be in order, since some healthy embryos may not survive to day 5 and you wouldn’t want to risk losing them.
Day-3 embryos are judged primarily on three criteria. First is the rate of cell division. An embryo that has seven, eight, or nine cells is more likely to implant than an embryo with either fewer or more cells. “More than 10 is not a good sign. It means the embryos are developing faster than they should,” says John Garrisi, Ph.D., laboratory director at the Institute for Reproductive Medicine and Science of Saint Barnabas in New Jersey. “Embryos with four or five cells on day 3 also have a low chance of implantation, about 10 to 15 percent, or about half that of a seven- to nine-cell embryo, depending on the patient.”
Embryologists next assess whether the cells are symmetrical in both size and shape. “An embryo that divides unevenly may not be up to snuff,” Dr. Go says. The third trait considered is fragmentation, the percentage of the embryo that has broken off into pieces during cell division. “Anything under 20 percent fragmentation is a good embryo,” says Denny Sakkas, Ph.D., director of the embryology laboratory at the Yale Fertility Center in New Haven, Connecticut. “Twenty to 40 percent is medium-quality. Fifty percent is poor.” If a patient has no other embryos, though, some labs will transfer ones that have up to 50 percent fragmentation.