The idea of having multiple babies at the same time at first might sound tantalizing. One grueling nine-month period and voilà: You're on your way to synchronized play dates and kids helping each other with homework. The reality is, if you birth multiples, meaning you house more than one baby in your womb, you are less likely to end up appearing on TV and scoring a book deal than you are to end up grappling with complex health issues that last a lifetime. It's worth it, then, to take a look at how multiple births occur, and how having one can affect a family in the short term and over time.

How are multiple pregnancies conceived?

Only a fraction of of multiples are conceived naturally, says William Schoolcraft, M.D., founder and medical director of the Colorado Center for Reproductive Medicine, a Denver fertility clinic widely regarded as one of the best in the world. When we think of a multiple birth, most of us think of in vitro fertilization, but the majority of multiple births, especially those with triplets or higher, are the result of injecting fertility drugs, says Schoolcraft.

In vitro offers a physician more control over how many eggs are implanted in a woman's womb. Clomid, a fertility drug taken orally, increases the likelihood of a twin pregnancy. A pregnancy with triplets, quadruplets, or more is usually the result of injected fertility drugs, along with sexual intercourse or artificial insemination.

"With injected fertility drugs, you can get lots of eggs, perhaps 10 eggs or more, and you really don't have control of how many eggs are produced," says Schoolcraft. "You can look and think you see three or four follicles, but there may be a couple more hiding. [They're] easy to miss." Because of this margin of error, many physicians steer clear of injecting fertility drugs, and instead use Clomid or in vitro. "More and more, doctors are shying away from these injectable drug treatments," says Schoolcraft.

Are multiple births regulated?

While no laws exist in the U.S. to dictate the occupancy limits of a woman's womb, the American Society of Reproductive Medicine has established clear guidelines by which reputable clinics and physicians abide. If a doctor is a member of ASRM, then he or she agrees to follow these standards. The standards are age-based and determined by the likelihood of successful embryo implantation. (The older you are, the more likely it is that you will need to try with more embryos to result in a viable pregnancy.) The ASRM recommends:

  • Women under 35 should ideally receive one embryo, but no more than two embryos
  • Women between 35 and 37 are to receive no more than two or three embryos
  • Women between ages 38 to 40, three or four embryos
  • Women older than 40 years, no more than five

"So with a 33-year old, you are going to be more careful, because her chance of keeping all embryos [is] greater," says Schoolcraft. "At the age of 41, twins, or more, are uncommon even with the transfer of multiple embryos."

It's a numbers game, and no one can be sure of the outcome. "We have to balance the desire to get pregnant against the risk of multiples, and at the same time we have to make adjustments for age," says Schoolcraft. "Our goal is to avoid triplets at all cost. We try to avoid twins, but the [health] risk [faced by] twins isn't nearly as great."

The ASRM guidelines used to recommend that physicians implant more embryos. That was until 1998, when Schoolcraft's clinic pioneered a method of in vitro fertilization called blastocyst transfer. Using this technique, doctors are able to grow embryos in a petri dish for five days, a longer period of time than had been previously possible. After the five-day time period, it's easier to identify which embryos are most viable, allowing a higher success rate of implantation—thereby lowering the need to implant more embryos than ideally wanted by the mother.

A focus on one healthy embryo

"Most researchers focus on identifying ways to access the best embryo, so we can make one viable pregnancy. We're getting close to the day where we can just put back one embryo and have success."

A woman in the U.S. would receive more than the regulated amount of implanted embryos only if her physician identified a special need. "Rarely, exceptions are made," says Schoolcraft. "You have to make a note in the chart, explaining what are the medical circumstances justifying this altered judgment."

"An octuplet scenario is extreme," says Schoolcraft.

Worldwide, in vitro standards differ from those in the U.S. "In Europe, there are many countries where the government pays for in vitro, and because they pay for it, they can regulate it." In many European countries, the governments allow a physician to implant only one embryo at a time, therefore the pregnancy success rates from in vitro are lower in Europe than in the U.S., says Schoolcraft. In more developing countries, where technology is less sophisticated and there are fewer regulations, physicians are implanting more embryos than what is recommended in the U.S., he says.