In cases of which a woman has conceived and has had multiple embryos transferred to the uterus. She may be at risk of multiple pregnancy. Pregnancies that are twins, triplets, quadruplets, and more are at higher risk of pregnancy loss and perinatal complications as compared to other pregnancies in which only one embryo was implanted. And these cases the woman has the option to undergo what is termed a selective reduction. What that means is that, in cases in general, in which there are more than two embryos implanted and sometimes in cases where it's only two embryos implanted a reduction can be done by placing a needle into the fetal sac and then injecting chemicals that instantaneously stop the heart from beating.
This is performed typically at around 9-12 weeks of pregnancy. It is done at this late time because naturally reduction occurs at such a high percentage cases that many times that the actual reduction does not have to be perform by the physician. Selective embryo reduction is emotionally challenge for couples to do, and it is a situation that couples should not have to face very often. What is required is that the couple and the physician make a good decision initially about how many embryos to transfer into the uterus. If the couple has adverse feelings about reduction they should never be place in that position. Single embryos or two embryos could potentially be transferred into the uterus at the time of embryo transfer reducing the need to ever face the situation of selective reduction. If selective reduction is required, however, it is generally safe for the remaining embryos.