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double duty

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Contents

Chapter One

Parents of twins hear all sorts of odd questions and misinformed comments about twinning from friends and family (and even strangers), such as "Are they paternal twins?" and "I thought all same-sex twins were identical!" High school science class covered the basics, but most folks have forgotten why and how this phenomenon happens. What follows in the next few pages is a crash course in Biology 101 so that you may go out and reeducate the world on the miracle of twinning. Now, class, pay attention....

Just the Stats, Ma'am

A staggering 95 percent of all multiples are twins, and more than 125 million sets roam the world today. The number of multiples, particularly twins, has been on the rise since the early 1980s, when doctors first learned how to make women superovulate, or release multiple eggs in one cycle. In 2005 (the most recent year for which statistics are available), the United States reported more than 133,122 live twin births, compared with approximately 110,670 in 1998 (the year Double Duty was first published). Yet the number of twins was only 68,339 in 1980. That's a 70 percent jump in 25 years. To bring those numbers a bit more in focus—during the 1980s, twins occurred in one in every 80 to 90 live births; in the late 1990s, it was one in every 40 to 45 live births. Today, twinning happens in about one in every 33 live births.

And what about other countries? Has twinning been on the rise around the world as well? You betcha! Canadian twin births, for instance, have also spiked through the years, rising more than 30 percent from 1974 to 1990. In the Netherlands, twinning has soared nearly 65 percent from 1970 to 1995. Australia, England, France, Denmark, Brazil, South Korea, Japan, and many other countries have also seen their share of double strollers these days.

Why the Increase?

The explosion of twinning is due to a number of complex biological, medical, and environmental factors. Some are well known and well documented; others are just speculative at best (but fun to read about nonetheless).

Fertility Drugs

During the late 1970s, fertility drugs became widely available to couples who had problems conceiving due to ovulation disorders. Clomiphene citrate (Clomid), the most common of all ovulating-inducing drugs, stimulates a woman's hypothalamus and pituitary glands to pump out more hormones, which in turn fuel the ovaries to produce and release more eggs at the time of ovulation. A woman who takes clomiphene citrate increases her chances of twinning by 10 percent. (Although some doctors say the percentage is much higher—especially if a woman already ovulates on her own.) A woman's likelihood of a multiple birth rises even further if she takes more potent, injectable fertility drugs such as human menopausal gonadotropin (Perganol), human chorionic gonadotropin (Pregnyl), or follicle-stimulating hormone (Fertinex).

Assisted Reproductive Technology

In vitro fertilization (IVF), the most common of all assisted reproductive technology (ART), has been helping infertile couples conceive for more than 25 years now. It involves removing eggs from the mother (following a round of ovulating-inducing drugs), fertilizing them in the laboratory using her partner's sperm, and then implanting the embryos back into her uterus. Several are implanted at once to increase the likelihood of success. In recent years, however, the procedure has become so successful that two, three, or more embryos are often viable, contributing to the explosion in multiples births. (One report published in the journal Twin Research back in 2003 noted 40 percent of IVF babies results in twinning.) To help stem this tide, many in the medical community are calling for single embryo transfer (SET), especially for women under the age of 35 who have a longer time frame for conceiving. It may be working, too, as the year-over percentage increase in twinning is not nearly as dramatic as it has been in the past and is finally leveling off.

One mom who was interviewed for this book got pregnant with twins through IVF after having only two embryos implanted. She was shocked since just a few years earlier she conceived a singleton, a son, after transferring a whopping 10 embryos and "hoping for a miracle."

Age

A woman who delays childbearing until after age 30 is also at a greater risk of twinning. As a woman gets older, she produces higher levels of gonadotropins (peaking between ages 35 and 39), hormones that stimulate the ovaries to release more eggs. Two of these hormones are the follicle-stimulating hormone (FSH) and luteinizing hormone (LH), both responsible for ovulation. A 35- to 40-year-old woman is three to four times more likely to have twins than a woman who is 20 to 24 years old. Older moms have clearly had an impact on the rise in twinning. In fact, more twins were born to moms aged 45 to 49 in 1997 than during the entire decade of the 1980s.

Birth Control Pills

A woman who gets pregnant shortly after the cessation of certain types of birth control pills has a greater chance of conceiving twins as well, although the results have been found to be inconclusive in some studies. (Of the 39 moms interviewed for this book, one mom of identical boys conceived almost immediately after stopping birth control pills. Coincidence? Perhaps.)

Inherited Trait

Yes, twins do run in families, but only fraternal twins (not identical) and only through the female bloodline. Some women inherit a gene for hyperovulation, or the predisposition to produce multiple eggs in a single cycle. So if your mom is a fraternal twin, or has a set in her family, that could explain why you are expecting twins.

Women Who Already Have Had Children

Due to hormonal changes, the more children a woman has, the more likely she will conceive twins with each successive birth.

Women Who Already Have Had Fraternal Twins

Indeed, lightning can strike the same place twice. A woman who has already given birth to fraternal twins without the use of fertility drugs doubles her chances of repeating it again with her next pregnancy since she's already proven that she's fecund, or able to produce abundant offspring!

Race

African races (especially West Africans) have the highest incidence of twinning (they are about 20 percent more likely than Caucasian women to have twins), followed by Europeans, and then Mexicans. Asian women have the lowest incidence.

Body Type

If a woman is tall and heavy with a body mass index (BMI) of 30 or above, her chances of having fraternal twins is significantly greater than a woman with a BMI of 20 or less since overweight women tend to produce more FSH.

Diet, Environment, and Other Factors

If you search hard enough, you can find a host of other theories that explain the rise in twinning. The Journal of Reproductive Medicine, for instance, reported in 2006 that women who regularly eat dairy products are five times more likely to conceive fraternal twins than women who don't. Researcher and author of the study, Dr. Gary Steinman, theorized that the synthetic growth hormones given to some diary cows could be the culprit....