How Do Twin Pregnancies Happen?
According to estimates by the US-based Centers for Disease Control and Prevention (CDC), the birth of twins has increased by nearly 72 percent between 1980 and 2018. As of 2018, there are nearly 33 sets of twins born per 1000 births in the United States.(1, 2) This means that twin births have almost doubled over the last 40 years. A woman has a greater chance of having twins if she conceives with the help of fertility treatment and is aged 35 years or over. In the United States alone, fertility treatments account for over one-third of twin pregnancies.(3, 4, 5, 6)
But why do twin pregnancies happen? Well, doctors don’t really completely understand the exact reasons behind why twin pregnancies sometimes happen. However, there are certain factors that can increase the chances of giving birth to twins. These include:
- Having fertility treatments
- The woman’s age at the time of conception
- Having a family history of twins.(7)
Conception takes place when a sperm fertilizes an egg to form an embryo. However, in case there are two eggs present in the womb at the time of fertilization, or the fertilized eggs end up splitting into two separate embryos, it is possible for a woman to get pregnant with twins.
Now all twins are the same, though. There are two types of twin conception that can happen:
- Identical twins: A pregnancy that leads to identical twins happens when a fertilized egg splits into two separate embryos. These embryos are known to be monozygotic. This means that these embryos have identical genes. Identical twins are usually the same sex as each other and also look very similar.(8)
- Fraternal or Non-identical twins: A pregnancy with fraternal twins can happen when two eggs are present in the womb at the time of fertilization, and the sperm fertilizes them both. These embryos, though, are dizygotic. This means that the two embryos do not contain the same genes, and they are not necessarily of the same sex also.(9)
Fraternal twins are much more common in women who undergo fertility treatment because doctors usually choose to place two or more fertilized embryos in a woman’s womb in order to increase the chances of having a successful pregnancy.
How Common are Twin Pregnancies?
Twins are not that common. According to data from the American Society for Reproductive medicine (ASRM), only about one in 250 pregnancies results in twins. However, the birth of twins has increased quite a bit as the use of fertility treatments has gone up, along with more women opting to have children at a later age. Since the 1980s, the birth rate of twins has increased by over 75 percent.(10)
A woman is more likely to have fraternal or non-identical twins as compared to identical twins. Identical twins account for just one-third of all multiple pregnancies.(11)
Can You Have Twins Naturally?
There are two ways to conceive twins naturally:
Conceiving Identical Twins
The first process involves a single egg getting fertilized by a single sperm. While this is the very basics of reproduction, but something happens to this process that leads to the creation of twins. Somewhere along the way, after the egg has been fertilized, the fertilized egg divides by itself into two, resulting in identical twins. The chances of having identical twins, though, are very rare. Only three or four births in every 1000 births result in identical twins.(12)
Identical twins are always the same sex, either both boys or both girls. Not only do they look alike, but they also share the exact same DNA.
Conceiving Fraternal Twins
Fraternal twins happen when two separate eggs get fertilized by two different sperm cells. Both these fertilized eggs go on to implant into the uterus, and two babies are born at the end of the pregnancy.
Fraternal twins can either be both boys, both girls, or one boy and one girl. They may or may not look alike. This is because, unlike identical twins, fraternal twins do not share the exact same DNA. In fact, apart from their age, they are typically no more similar than those siblings who are born years apart.
Factors that Increase the Odds of Having Twins Naturally
There are several factors that increase the odds of conceiving twins. Here are a few of them.
It is common to hear that twins tend to run in families. Well, this statement is only partially true. The chances of having fraternal twins are higher if you yourself are a fraternal twin or if fraternal twins run in your family.
One of the most common reasons for this is hyperovulation. Hyperovulation is a condition where the body releases two or more eggs regularly during ovulation. Having at least two eggs is one of the requirements for having fraternal twins.
Hyperovulation is something that can be passed down in the DNA. You can get it from either your mother’s side or even from your dad’s side. However, hyperovulation can also happen sometimes in women who don’t usually release more than one egg or have twins in the family.(13)
Research by the American Society for Reproductive Medicine (ASRM), women who themselves are fraternal twins can produce twins in approximately 1 in every 60 births. For men who are fraternal twins, the chances of having twins are 1 in every 125 births.(14)
Many people also believe that twins can sometimes skip one generation. This means that a person may be likely to have twins if one of their grandparents did. However, as of now, there is no evidence to support this theory.
If you are over the age of 35 years and you want to try to have twins, you are more likely to successfully conceive twins if you are in your late 30s or early 40s. It has been found that women who are at an advanced maternal age have a greater chance of having twins.(15)
Hormonal changes that happen the closer you get to menopause are believed to encourage the body to release more than one egg during ovulation. So if two or more eggs get fertilized and both implant in the womb, you may end up having twins.(16)
Height and Weight
Studies have also shown that taller women have a higher rate of having twins. While this sure sounds strange, but researchers have found that a certain insulin-like growth factor in the body of taller women might be the cause of this possibility. A study carried out in 2006 looked at the heights of 129 women who had spontaneous multiples and found that the average height of these women was around 5 feet 5 inches. The national average at the time of all women was 5 foot 3 3/4 inches.(17)
It is also believed that women who are overweight or obese also have a higher chance of conceiving twins. The reason behind it remains unclear, but it is believed that nutrition may have a role to play. Women with a greater body weight are likely seen to have more resources available for a developing fetus.(18)
According to a 2005 study that explored data from over 51,000 births in the United States, the chances of having twins are especially the highest in women who have a body mass index (BMI) above 30.(19)
Another study carried out in 2016 analyzed various maternal characteristics amongst women in Norway. The study found that rates of having twins increased in women who had a BMI above 25.(20)
However, this does not mean that you should gain weight intentionally in order to boost your chances of having twins. Remember that having a BMI over 30 also places you in the high-risk category of pregnancy, which is why it is important to talk to your doctor about maintaining a healthy weight before you get pregnant.
Another older study by the Long Island Jewish Medical Center who also reported on the link between the maternal height factor and twin births found that women who are vegans (do not consume any dairy products) give births to twins at an astounding rate of 1/5 times as compared to women who consume dairy products.(21)
This may be because women who eat dairy tend to consume extra insulin growth factors. Cows release this hormone into their milk, and when women consume the milk, it may have an impact on reproduction. However, this is just one analysis based on medical records of women who had given birth. Little to no research is there on how consumption of dairy affects the twin birth rate.
In 2008, a study focused on women giving birth in southwest Nigeria. Nigeria is known to have the highest rate of twin births in the world. The analysis noted that eating yams might be increasing the likelihood of having twins among the Yoruba population in the area. The common belief is that yams are rich in the natural hormone phytoestrogen, which might be causing multiple ovulation.(22)
However, while increasing your intake of yams is not a guaranteed way of having twins, but eating yams, which is a root vegetable rich in many vitamins, minerals, and fiber, will also not harm you in any way.
Birth statistics from 2018 in the United States have shown that black women deliver twins at a higher rate than white women. At the same time, Hispanic and Asian women deliver twins at a lower rate than the other racial groups.(1)
If you already have a child, then this could be a reason behind having twins. A previous pregnancy, or better still, if you have had multiple pregnancies in the past, it may increase your chance of having twins.
Women who are between the ages of 35 to 40 and have had four or more children in the past are nearly three times more likely to have twins as compared to a woman under 20 years who has had no previous pregnancies.
The reason behind this is not precisely known, but it is believed that it is probably because, with each pregnancy, you are getting a bit older.
Having Twins Through Fertility Treatments
Couples who undergo fertility treatments like in vitro fertilization (IVF), artificial reproductive technology (ART), and other procedures like intrauterine insemination (IUI) are more likely to have twins.
Intrauterine Insemination and Twins
Even though the process of IUI by itself does not increase the chances of having twins. There are some medications associated with the procedure that might boost the likelihood of having twins. For example, clomiphene citrate (brand name Clomid) and letrozole (brand name Femara) are ovulation stimulating medications that are often prescribed in IUI cycles to help the body to produce multiple eggs that are likely to release at the same time. If two or more eggs get fertilized and implanted at the same time, twins or multiples are a possibility.
In a 2014 study, it was found that the rate of twins in women taking Clomid was 7.4 percent. On the other hand, Femara had a lower rate at only 3.4 percent.(23) These numbers might not appear to be very high, but they are still considerably higher than the chance of being able to conceive twins naturally.
Additionally, gonadotropins like follicle-stimulating hormone (FSH) are also known to boost the growth of egg follicles. These are injectable medications that are also frequently used in IUUI procedures and other fertility treatments. The rate of twins in women who are using these medications is a whopping 30 percent.(24)
In Vitro fertilization and Twins
Medications like Clomid and others are also a part of IVF. However, in this type of reproductive technology, the factor that increases the chances of having twins is the number of embryos that are transplanted. Some couples decide to transfer just one embryo. Now it is highly unlikely that a single embryo will split and turn into identical twins.
The likelihood of having fraternal twins or multiple babies happens when couples decide to transfer two or more embryos, and they successfully impact and go on to develop into two or more fetuses.
The rate of having twin pregnancies with IVF with the use of fresh embryos is 12.1 percent for women under the age of 35 years and 9.1 percent for women who are between 35 to 37 years. The chances further go down with age, and women in the ages of 38 to 40 only have a 5.3 percent rate of twins, and for women who are 43 and older, the rate is hardly 0.5 percent.(25)
How to Increase the Odds of Having Twins?
Before you start planning twin pregnancies, it is important to keep in mind that being pregnant with multiples also carries certain complications and will automatically put you within the high-risk pregnancy classification. Women who are pregnant with twins are at a greater risk of developing gestational diabetes and preeclampsia.(26) While this does not mean that you won’t have a healthy pregnancy, it just means that you would need to be monitored more closely during your pregnancy.
However, there are a lot of factors that increase the chances of having twins, but they are not in your control. While you can increase your intake of dairy products and yams, but there is no way that you can change your family history, race, and height. Even gaining weight on purpose before planning your pregnancy is not necessarily going to help. Furthermore, if you are depending on having kids at a later stage in life to increase the chances of having twins, it is important to understand that with advanced age, there is also the risk of reduced fertility and a higher risk of chromosomal abnormalities in the fetus.
If you really want to have twins, using reproductive technologies like IVF and others gives you the best chance. Ovulation-enhancing drugs, if used alone or with IUI, also require you to have a prescription and may even carry some serious risks, including ectopic pregnancy.
Drugs and reproductive procedures like IVF are also costly and usually reserved for couples who have infertility problems.
Unfortunately, there is no sure shot way in which you can guarantee that you will have twins. However, you can try to increase your odds by having roasted yams or increasing your consumption of dairy products. A twin pregnancy comes with its own risks and rewards, so keep those in mind before you go ahead.
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- Cdc.gov. 2021. ART and Multiple Births | Key Findings | Assisted Reproductive Technology (ART) | Reproductive Health | CDC. [online] Available at: <https://www.cdc.gov/art/key-findings/multiple-births.html> [Accessed 13 November 2021].
- Pinborg, A., 2005. IVF/ICSI twin pregnancies: risks and prevention. Human reproduction update, 11(6), pp.575-593.
- Moise, J., Laor, A., Armon, Y., Gur, I. and Gale, R., 1998. The outcome of twin pregnancies after IVF. Human reproduction (Oxford, England), 13(6), pp.1702-1705.
- Charnock, W. and Nutting, J., 1967. The factors determining the frequency of occurrence of annealing twins. Metal Science Journal, 1(1), pp.78-83.
- Agata, V.A., Couvert, N. and Pison, G., 2006. Trends in the Frequency of Twin Births over the last Century: European Comparisons.
- Taubman, P., 1976. The determinants of earnings: Genetics, family, and other environments: A study of white male twins. The American Economic Review, 66(5), pp.858-870.
- Kohler, H.P., Skytthe, A. and Christensen, K., 2001. The age at first birth and completed fertility reconsidered: Findings from a sample of identical twins. MPIDR Working Paper, WP-2001-006.
- Machin, G., 2009, May. Non‐identical monozygotic twins, intermediate twin types, zygosity testing, and the non‐random nature of monozygotic twinning: a review. In American Journal of Medical Genetics Part C: Seminars in Medical Genetics (Vol. 151, No. 2, pp. 110-127). Hoboken: Wiley Subscription Services, Inc., A Wiley Company.
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- McLennan, A.S., Gyamfi-Bannerman, C., Ananth, C.V., Wright, J.D., Siddiq, Z., D’Alton, M.E. and Friedman, A.M., 2017. The role of maternal age in twin pregnancy outcomes. American journal of obstetrics and gynecology, 217(1), pp.80-e1.
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