Delaying motherhood and the introduction of fertility treatments are the two biggest drivers behind the rise in twin births since the early 1980s.
Let's talk about advanced age first. Women may postpone having children for any number of reasons; some commonly cited are career, financial security, emotional stability and some women are waiting for the right partner. Some women may need to delay motherhood for health reasons. There's a problem with postponement, though: Your body might be keeping a different schedule that it did when it was younger. As a woman ages, the quality and quantity of her eggs declines, a decline that begins around her 30th birthday and take a big drop by her late 30s. Women who postpone pregnancy until their mid-to-late 30s or into their 40s are considered advanced-aged pregnancies. As a woman's fertility declines, she is less likely to become pregnant naturally, and her risk of suffering a miscarriage if she does conceive increases.
Fertility treatments may help, and they may be helping to increase the odds of us all seeing double. According to the American Society for Reproductive Medicine (ASRM), the risk of conceiving multiples is greater than one in three for successful pregnancies resulting from fertility treatments [source: ASRM]. This is because the treatments stack the deck, so to speak. Fertility drugs encourage a woman's body to produce multiple eggs each cycle, and multiple embryos are often implanted during in vitro fertilization.
Fertility treatments may increase the odds of having a baby when the odds seem against you, and they have become a family-building choice for many parental hopefuls. Individuals who are working against time also benefit from assisted reproductive technology. Between 2007 and 2009, the number of babies born to women under the age of 40 declined overall, but the birth rate rose as much as 6 percent for women ages 40 to 44 (which is about 3 percent of the total number of births) [source: CDC].
Babies conceived with assisted reproductive techniques and other fertility therapies such as artificial insemination and drugs used to stimulate the ovaries have been increasing since 1978, when Louise Brown was the first baby born as a result of in-vitro fertilization (IVF), a type of ART procedure. Since Brown's birth, more than 3 million babies have been born resulting from ART, and between 1980 and 2004 the number of babies born to women over the age of 40 has quadrupled [sources: Moisse, Newman].
It's not only a woman's age and prior pregnancy history that can impact her likelihood of having non-identical multiples. Her diet, height and weight and her genetic disposition can all affect her pregnancies. There is also limited evidence that women who take birth control pills may have an increased rate of conceiving fraternal twins in the two months after they stop taking the contraception [source: Good Morning America].
Let's look more closely at the risks that having multiple babies poses during pregnancy, and if there are risks for babies who share a womb.