The national average for women younger than 35 able to become pregnant by in-vitro fertilization (IVF) on the first try (meaning, the first egg retrieval) is 55%. However, that number drops steadily as the woman ages.

Ovarian stimulation is used to mature multiple eggs for egg retrieval. Even if ovulation is normal, fertility drugs are used to produce more than a single egg because pregnancy rates are higher with more eggs. An average of 10 – 20 eggs are usually retrieved for IVF.

Between one-third and one-half of all fertilized eggs never fully implant. A pregnancy is considered to be established only after implantation is complete.

One of the most common reasons as to why an IVF cycle fails is due to the quality of the embryo. Many embryos are unable to implant after transfer to the uterus as they are defective. Embryos that look healthy in a lab may have defects that cause them to die rather than grow.

When such eggs are fertilized, it leads to the generation of embryos which are genetically incompetent. Either such embryos do not implant and even if they do, the pregnancy ends in early miscarriage. In rare instances, they can also lead to a full-term birth where the newborn has genetic defects.

It’s rare for IVF patients to bluntly request twins, and few ask for triplets or more, but many mention a desire for twins, IVF doctors tell WebMD. That happens “all the time,” says Mark Perloe, MD, medical director of Georgia Reproductive Specialists in Atlanta.

This is why IVF centers stimulate women in order to get sufficient eggs. Women under 38 in our IVF program have acceptable live birth rates even with only 3 – 6 eggs, do better with more than 6 eggs, and do best with more than 10 eggs.