Under normal conditions, it almost never does. Women’s hormonal rhythms are choreographed to ensure that the ovaries release one–and only one–fertile egg every month. Early in the menstrual cycle, a drop in circulating estrogen and progesterone prompts the body to release other hormones (GRF, FSH and LH). When stimulated by those hormones, egg-bearing sacs, or follicles, within the ovaries start to swell. After a week or so, a follicle will mature to the point where it’s ready to release its egg into the fallopian tube for fertilization. But before doing that, it spurts estrogen into the bloodstream–shutting down the cascade of ovulation hormones and halting the growth of other follicles.

Fertility treatments can easily disrupt that arrangement. When supercharged by a drug like Pergonal or Metrodin (the treatment McCaughey received), a woman’s ovaries can release as many as 40 eggs in one cycle. Doctors may extract them for in vitro fertilization (IVF) or leave them to be fertilized naturally. Though multiple births are rare under normal conditions, a third of all IVF pregnancies, and up to 20 percent of those achieved with fertility drugs, involve two or more babies.

That wouldn’t be a problem if humans had the carrying capacity of, say, cats. But they don’t. Women carrying multiple fetuses risk anemia, hypertension and labor complications that can require Caesarean delivery. And their babies are often born prematurely. On average, each additional fetus shortens the usual 40-week gestation period by three and one-half weeks. A pair of twins born at 36 weeks may do fine. But as the number of fetuses increases, the kids’ gestation times and birth weights decline.

So do their health prospects. Even if they’re born alive, triplets, quadruplets and quintuplets are 12 times more likely than other babies to die within a year. Many suffer from respiratory and digestive problems. They’re also prone to a range of neurological disorders, including blindness, cerebral palsy and mental retardation. Until a fetus reaches about 36 weeks, its developing brain maintains a delicate cell factory known as the germinal matrix. The matrix normally finishes its job before a child is born. But if it’s still active at the time of delivery, normal fluctuations in blood pressure can cause a hemorrhage that not only stalls neuron production but injures existing brain tissue. By selectively reducing the number of fetuses in a woman’s womb–i.e., killing two of four quadruplets by lethal injection–a specialist can avert such problems. But couples can’t always stomach that prospect, so doctors are often left struggling to postpone a dangerous delivery as long as possible.

These crises are not inevitable. After giving a woman fertility drugs, specialists can use imaging tests to monitor the behavior of her ovaries. If the tests show that numerous eggs have been released, the doctors can advise against sex or sperm injections and try a lower dose on the next cycle. And in newly adopted guidelines, the American Society for Reproductive Medicine agrees that IVF specialists should limit the number of viable embryos they place in patients’ wombs. But fertility clinics are unregulated, and as long as their clients demand success at any cost, the boom in multiple births is likely to continue. Would-be parents should realize that more is not always merrier.

Women’s hormonal rhythms normally ensure production of just one viable egg every month. By forcing the release of numerous eggs at once, fertility drugs raise the chance of multiple birth.

The Normal Ovary

Under ordinary conditions, an ovary is the size of a grape. Inside there are fluid-filled sacs called follicles, each housing an egg, or ovum. During normal ovulation, one follicle matures, releasing its egg for fertilization in the fallopian tube.

The Ovary on Drugs

Fertility drugs jump-start follicle development, increasing the likelihood that more than one follicle will release a fertile egg. Drug treatment can make the ovaries swell up to 10 times their normal volume–roughly the size of a grapefruit.

Multiple Birth

The human uterus isn’t designed to hold numerous fetuses. When it is forced to, the crowding causes early delivery. The consequences for babies can range from brain damage to death.