How to Have Twins: the Myths and the Science

Curious to know how to get pregnant with twins? Here’s what experts know so far.
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profile picture of Victoria Veilleux
Published September 8, 2017
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People have an insatiable curiosity when it comes to how to have twins. No, I didn’t conduct some groundbreaking metadata analysis. I know this firsthand as a mom of 7-year-old twins. Nearly every day since getting pregnant with twins I’ve been asked if twins run in my family. Apparently, people want to know what are the chances of having twins and even (much to my surprise) how to have twins.

Truth is, I got pregnant with twins “naturally,” though the term in twin circles (yes, there is a twin subculture with its own jargon) is “spontaneously.” I didn’t necessarily want or not want twins. It just happened. Since their birth—because I am a health writer and because I wanted my answer to be more than “yes, twins run in my family”—I’ve done plenty of research on the topic. Not surprisingly, I haven’t come across any over-the-counter pills you can take to get pregnant with twins or sex positions to conceive twins (people have asked). However, I did talk to some of the leading experts on the topic of how to have twins and scoured the research to shed a little light on how and why twins happen. Here’s what I discovered.

How Do Twins Happen?

Figuring out how to have twins starts with understanding how twins happen. Despite the amazing advances in computer imaging and genetic testing, how to have twins, in many respects, still remains a mystery. What we do know is that there are essentially three major things that happen when sperm meets egg and forms a zygote: The inner membrane gets filled with amniotic fluid, which eventually protects the fetus in the womb. The outermost protective membrane around the embryo becomes the chorion, which develops a supply of blood vessels. And the chorion works with the uterus lining to form the placenta, which ultimately supplies oxygen and nutrients to the fetus, among other things. This zygote typically develops into a single embryo, which turns into a fetus and, eventually, your baby.

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So, how to have twins? Twins happen when the situation veers a little offtrack from the above norm. Identical twins (aka, monozygotic twins) are the result of one egg being fertilized and then splitting into two embryos. Many experts believe this occurs as a result of some kind of cell abnormality, possibly resulting from a calcium deficiency that weakens the protein wall that holds the cell together.

Despite common belief, identical twins don’t always look exactly alike. Sure, indicators like shared eye and hair color, the same blood type, ear shape, even the order in which they cut their teeth can be indicative of identicals. However, developmental differences can happen in the womb, depending on where their umbilical cords implant—like how evenly they receive oxygen and nutrients from a shared placenta, for instance. This, in turn, can result in different heights and other features.

Many people also think that identicals always cozy up together in one happy bath of amniotic fluid, but this isn’t the case, depending on when the embryo splits up and how. There are actually quite a few types of identical twins, but roughly two thirds of them are known as monochorionic-monoamniotic twins (commonly referred to as mono-mono or MCMA twins). These twins have their own amniotic sacs in the womb but share the placenta and chorion. About another third are known as dichorionic-diamniotic identicals (commonly referred to as di-di or DCDA twins). While the vast majority of these twins occur when two eggs are fertilized by two sperm simultaneously (see below), they can also originate from one sperm and one egg, which then form into a zygote that splits into two—giving each twin its own amniotic sac, chorion and placenta.

Fraternal (dizygotic) twins happen when two eggs are being fertilized by two different sperm. They are simply siblings born at the same time. All fraternal twins are di-di twins, meaning that they both have their own placenta, amniotic sac and chorionic sac. Except with rare scientific anomalies, boy-girl twins immediately tip you off that the twins are fraternal. (Every mom of mixed-sex twins have a running joke about being asked if their twins are identical or fraternal.) The same goes for same-sex twins who look dramatically different—like my daughters, who each have different hair and eye color.

How Common Are Twins?

According to the Centers for Disease Control and Prevention’s National Center for Health Statistics, the odds of having twins in 2015 (the most recent figures available) were 33.5 for every 1,000 births in the US, with only four of those births being identical twins.

It’s no surprise that the number of twins skyrocketed with the introduction of fertility treatments. With in vitro fertilization (IVF), for instance, several eggs are extracted from the ovaries and brought together with sperm in the lab, in the hopes that at least one embryo will form; if more than one forms, then more than one is typically placed in the uterus to increase the chances of at least one implanting and turning into a healthy fetus. Sometimes, though, more than one actually does—which explains why twinning rose a whopping 76 percent from 1980 to 2011, as fertility treatments became more common.

After removing the IVF factor, how common are identical twins? Not much at all. The chances of having twins of this kind prove fairly consistent worldwide—around 4 for every 1,000 births. However, fraternal twin statistics vary all over the map, with the highest rate of dizygotic twinning found among the black populations of Africa: 45 per 1,000 in an area of Nigeria. The lowest odds of having twins of the fraternal sort is in South East Asia and Latin America.

If you’ve been wanting to know how to have twin boys, the odds aren’t in your favor. Girl twins are more common—which is the opposite case of singleton births in the US, where there are 105 boys born for every 100 girls. According to the Washington State Twin Registry, this is because the death rate for males in the womb is slightly higher, and when you factor in the increased chance of death in the womb for twins, a higher number of female survivors, and therefore female twins, result.

What Are the Chances of Having Twins?

Your chances of having twins depends on a range of factors. But beyond genetics, they’re pretty speculative. The current research is either in early phases or resulting from what are called “randomized observational studies”—not a controlled one. This means that even though we might be noticing commonalities, it’s impossible to rule out other factors that might influence results. But because I know you’re curious—as was I—about how to have twins, here’s a peek at a few factors (outside of fertility treatments) linked with higher odds of getting pregnant with twins:

Twins on your mother’s side of the family. You are about twice as likely as the general public to have twins if you, your mom or your maternal grandmother was a twin. Now genetic studies are attempting to pinpoint which genes are responsible. One study boiled it down to two, and if a woman has both it increases her odds of having fraternal twins by 29 percent.

Twins on the dad’s side of the family. While the influence of mom’s genes is no news flash, the influence of dad’s is a more recent finding. One study found that 30 percent of twin dads had blood relatives who fathered twins. Higher levels of a protein called insulin-like growth factor (IGF-1) may be the reason, since it’s thought to enhance sperm speed, strength and count. Other studies have found that dads with more robust sperm are more likely to have twins, regardless of family history.

Older maternal age. Even though twins run in my family, just plain being old is the real reason I think I was blessed with twins at age 38. You are three times more likely to get pregnant with fraternal twins between the ages of 35 and 40 than between the ages of 20 and 25. I compare it to buying two drinks when it’s last call at the bar or stepping on the gas to beat a yellow traffic light. As you approach menopause, your body pumps out more FSH, a hormone that encourages the ovaries to release the last of their eggs—which can then result in releasing more than one egg. Interestingly, while high FSH might result in more twin births, it’s also associated with lower fertility, in general. That’s because we’re born with a set number of eggs, and as they reach their expiration date, they become less viable.

African heritage. Identical twinning appears to be color blind, but non-Hispanic black women have the highest chance of conceiving nonidentical multiples. Hispanics and Asians are on the other end of the spectrum with Caucasians falling somewhere in the middle.

Above-average height. If you’re over five foot, five inches your odds of having twins is looking up. One of Steinman’s studies found 129 moms of spontaneous twins to be on average more than an inch taller than the average height of US women. In addition, a review study of 32 countries showed that the countries with taller women also revealed higher rates for twins.

Very high or very low maternal weight. Obese women (those with a body mass index of 30 or higher) typically have a tougher time conceiving, but when they do, they have higher chances of having twins than women with a healthier pre-pregnancy BMI of between 19 and 25. What’s more, the increase of twins in the US has coincided with the rise of obesity in America. However, research also suggests that women under 118 pounds are three times as likely to conceive monozygotic twins, possibly due to low estrogen, which delays implantation and gives an egg more time to duplicate and split.

Breastfeeding. One study found that women who were breastfeeding when they got pregnant had a much higher rate of conceiving twins—as in nine times more likely! Gary Steinman, MD, coauthor of Womb Mates and a twin researcher (after having delivered a set of identical quadruplets in 1997), says this tends to happen around a year after the birth of the child they are still breastfeeding (which is a surprise to many women who think they can’t get pregnant during this window of time). His studies also revealed that the longer women breastfeed, the higher the chance of having twins in future pregnancies, though the reason remains unclear.

Previously having twins. As a woman gives birth to more children, so do her chances of having twins. For those who have already conceived fraternal twins spontaneously, brace yourself: Your odds of having twins again is quadrupled.

Getting pregnant in the summer. According to the Washington State Twin Registry, the most fraternal twins are conceived in July and the least in January. Frederick Naftolin, professor of ob-gyn at New York University, suggests that it might be a byproduct of diet. Various greens (and the things that you eat that ingested those greens, such as the cow who supplied your beef and milk) have different amounts of phytoestrogens, which can cause estrogen levels to rise or fall.

A diet rich in white Nigerian yam. Scientists don’t exactly see eye-to-eye when it comes to food and fertility, but there’s one food that seems especially linked to twinning (particularly in certain African populations): the white Nigerian yam. This yam—a staple in Southwest Nigeria, the twin capital of the world—contains phytoestrogens that seem to enhance the release of more eggs, either directly or indirectly. But don’t fill up your cart with the grocery-store variety of yam just yet. “This is not a regular sweet potato,” says Naftolin, the lead researcher of a well-publicized study on this root vegetable. “It’s the bark and trunk of a bush, which is pulled out of the ground, cut into slabs, and then the dried yam slabs are pulverized and made into patties and flaked off into breads and cereals.” It’s likely the preparation that increases the chances of having twins, since it doesn’t destroy the baby-boosting compounds.

Drinking a lot of cow’s milk. This theory is based on the idea that insulin-like growth factor (IGF), a protein that’s produced in the liver in reaction to growth hormone, may have something to do with increasing the chances of having twins. Steinman found that fixed amounts of milk increased IGF levels in the blood, and vegans, who consume no animal products (including dairy), had lower IGF than the general population. Knowing that higher levels seemed to be a big factor in twinning in cows, he examined that the rate of twinning among vegans was less than half of the general population. Increased levels of IGF in cow’s milk survives pasteurization and the casein in milk protects it from being digested in our stomachs—so IGF goes directly into our bloodstream, possibly triggering more eggs to be released. Cows injected with growth hormone in order to up milk and beef production have higher levels of IGF, he says, and in the US the rate of spontaneous twins has risen twice as much compared to countries where growth hormone injections are illegal. Another study in the ‘80s also revealed that twinning rose in tandem with the average amount of milk consumed in 15 different European countries. It’s just an observational study, but as a kid who grew up in Wisconsin (also known as America’s Dairyland) and is now a mother of twins, I find this link to be quite interesting.

How to Have Twins

Having a predisposition for a twin pregnancy is a scientifically valid phenomenon, but intentionally trying to improve your odds of conceiving twins? Not a good idea, experts says. Alice Domar, MD, director of integrative care at Boston IVF, tells me she has a conversation every day with someone who says they want twins, often because they already have fertility issues and because IVF is expensive or because they are older and want an insta-family. But she urges them to look at the risks. “A twin pregnancy is a higher-risk pregnancy,” she says. This includes increased chances of gestational diabetes and pregnancy-induced hypertension. In addition, more than half of twin births in the US are preterm, notes the March of Dimes. Stanford Children’s Health also notes that about 10 percent of twins have very low birth weight (defined as 3 pounds 4 ounces). So while you might get a “2-for-1” from an IVF standpoint, your copay for the NICU stay may more than make up for it.

Still, I can attest that, with twins, the joy at every stage of parenting is doubled, and, if you’re hoping to beat the odds, I get it. However, the smartest tips for how to have twins are those that simply enhance your fertility. So take a look, below, at what might improve your chances of becoming pregnant—and who knows, maybe you’ll have twins.

Be conscientious about the basics. Aside from keeping alcohol and caffeine to a minimum, maintaining a healthy weight, quitting smoking and not exercising to the extreme of messing up your menstrual cycle, Domar suggests taking a prenatal vitamin, and if you have an issue with egg quality, taking CoQ10. Several studies found as much as a 40 percent increase in twinning among women taking multivitamins with folic acid when they got pregnant. While other studies have questioned the findings, what’s certain is that folate (vitamin B9 ) helps prevent neural tube defects and can aid in DNA replication—a good thing for singletons as well as twins.

Be open to yoga and meditation. Even if scientists can’t say for sure that yoga improves fertility in both men and women by decreasing stress and balancing hormone levels, you might end up getting a nice dose of calm—something that will come in handy if you do end up getting pregnant with twins!

Mix it up in the bedroom. How to get pregnant with twins or even with just one child as a result of having sex every which way has been a hot topic for centuries. While any advice on sex positions to conceive twins is likely false, researchers are, in fact, exploring what else might affect fertility, beyond having sex when you’re most fertile. Computer-aided studies have ruled out male sperm being speedier and enhancing your odds of having twins who are male, but many findings contend the woman’s vaginal pH at different times of the month might make it tougher for those boy swimmers to beat out their female marathoners.

Consider acupuncture. For those undergoing IVF, a large 2002 study showed that women undergoing IVF who got acupuncture before their embryo transfers (with good-quality embryos) had a 42 percent pregnancy rate, while the control group had a 26 percent pregnancy rate. The 30 to 40 studies that followed, however, had mixed reviews as to whether the needles supplied more blood to the pelvis, decreased fertility-zapping stress or was even truly effective to begin with. (By the way, I was undergoing acupuncture for another condition when I asked my acupuncturist to put a needle into my fertility points just for prosperity’s sake. Next thing I knew, I was pregnant. With twins.)

How Do You Know If You Are Having Twins?

There’s no way to know whether you’re having twins until your first ultrasound around nine weeks, though two heartbeats and/or chorion sacs may be detectable as early as six weeks. If you happen to be monitoring your pregnancy early and closely (as fertility patients do), blood tests—which end up revealing higher amounts of hCG, the pregnancy hormone—could also give an indication.

It just so happens that hCG is the hormone that brings on morning sickness, so women pregnant with twins may be more likely to experience nausea and fatigue. They may also show a little sooner than singleton pregnancies. Still, every pregnancy is such an individual experience, so there are no dependable clues that you have two babies on board—or more!

Published September 2017

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