By Khadijat A. Olawuyi

In southwestern Nigeria, in the heart of Yorùbá land, sits a small town called Igbo-Ora. At first glance, it looks like any other lively town with busy markets, children playing, and families walking down the street. But if you spend just a few hours there, you will start to notice something unusual: you keep seeing double. Two identical toddlers holding hands. Two girls dressed alike. Two boys with the same smile. In Igbo-Ora, twins are everywhere.

The global average birth rate for twins is about 12 per 1,000 births. In Igbo-Ora, the rate is estimated at around 45 per 1,000 births, more than four times the global rate. Because of this, the town has proudly earned the nickname, “The Twin Capital of the World.” Every year, it even hosts a twin festival where twins from across Nigeria gather to celebrate. But behind the celebration lies a scientific mystery: why are there so many twins in Igbo-Ora? Studying the causes behind the unusually high rate of twins in Igbo-Ora could provide important insights into how ovulation and fertility are regulated, potentially improving reproductive health and fertility treatments in the future.

There are two main types of twins: monozygotic and dizygotic. Monozygotic twins occur when a single fertilized egg splits into two (Figure 1)¹. This type of twinning occurs at relatively similar rates across populations worldwide, unlike dizygotic twinning, which varies greatly between populations². Monozygotic twins are identical twins, sharing 100% of their DNA¹. Dizygotic twins, fraternal twins that share about 50% of their DNA, happen when the ovaries release two eggs during one menstrual cycle, and both are fertilized¹. This is called hyperovulation.  In Igbo-Ora, most twins are dizygotic³. So, the real question becomes: why do people with ovaries in this population release multiple eggs more often?

For years, many people believed the answer might lie in the diets of people living in Igbo-Ora⁴. Igbo-Ora is famous for dishes made with yams and okra. Some scientists have pointed to a compound in yams called diosgenin, which laboratories can use as a starting material to chemically synthesize steroid hormones such as progesterone. The belief of diet causing hyperovulation likely arose because, in the 1960s, pharmaceutical companies produced progesterone, androgens, and cortisone from compounds extracted from Mexican wild yams⁵. Hormones are chemical messengers that help coordinate processes throughout the body. In the reproductive system, hormones regulate the menstrual cycle by controlling when ovarian follicles —small sacs in the ovaries that contain immature eggs—grow and mature until the eggs are capable of being released and fertilized, when ovulation occurs, and whether an egg is released (Figure 2)⁶. Progesterone is especially important because it helps regulate the reproductive cycle after ovulation and prepares the uterus for pregnancy⁶. Since ovulation depends on tightly controlled hormonal signaling, some scientists wondered whether compounds related to steroid hormones such as diosgenin could influence the likelihood of releasing more than one egg during a cycle⁷. Although diosgenin can be industrially used to manufacture hormones, the human body cannot do the same—it cannot be converted from food into hormones able to be used in the reproductive cycle^8,9. To date, researchers have not found consistent differences in yam consumption between mothers of twins and mothers of single babies, and there is no clinical evidence that eating yams causes hyperovulation¹⁰.

Environmental factors can influence reproduction. Age, overall health, body weight, and even environmental exposures such as pollution, industrial chemicals, and differences in lifestyle can affect hormone levels and fertility¹¹. Living conditions and access to healthcare can also shape pregnancy outcomes¹². However, when scientists closely examined twinning rates, they found that the high rate of fraternal twins is not limited only to Igbo-Ora¹³, but also appears across the broader Yorùbá population, even in different regions¹³. When Yorùbá women move to other countries, their higher twinning rate often continues¹⁴. If the environment alone were responsible for hyperovulation, one would expect the twinning rate to decrease when people move to new places with different diets, climates, and lifestyles. The twinning rate remaining the same, despite people moving away from Igbo-Ora, suggests that while the environment may be an important factor, it is likely not the whole story¹⁵.

The genetic explanation is currently the strongest one. Genes are made up of sequences of DNA, the instruction manual our bodies use, that influence how the body develops and functions, including how hormones regulate reproduction¹⁶. During a typical menstrual cycle, several ovarian follicles containing immature eggs begin developing. Hormones, such as follicle stimulating hormone (FSH) ¹⁷, normally ensure that only one follicle fully matures and releases an egg during ovulation¹⁸. FSH helps immature ovarian follicles grow by stimulating the cells surrounding the egg to support its development and maturation until it is ready for ovulation (Figure 3)^12,19. As follicles grow, they also produce hormones such as inhibin B and estradiol which help reduce FSH levels and prevent additional follicles from continuing to mature^20,21. Fraternal twinning, which depends on hyperovulation, can occur when this balance changes and multiple follicles continue developing instead of only one¹⁸. Some women naturally produce slightly higher levels of FSH while others may have ovaries that are more sensitive to the hormone²². Fraternal twinning often runs in families, especially along the mother’s side²³. Certain inherited genetic differences may affect how the ovaries respond to hormones like FSH, increasing the chance that more than one egg matures during a cycle²⁴. Studies have identified specific mutations in genes such as FSHB and SMAD3, which are involved in FSH production and ovarian responsiveness to FSH, as potential contributors to fraternal twinning²⁴. About 20 percent of the variation in fraternal twinning appears to be genetic. When a population has a long history of high twinning, and that pattern persists even after people migrate to other countries, genetics becomes the most likely explanation.

A genome-wide association study (GWAS) published in Human Reproduction in 2024 analyzed genetic data from thousands of women²⁵. GWAS studies work by scanning the genomes (the complete set of genetic material or DNA found in an organism) of large groups of people and looking for genetic variants—small differences in DNA sequence between individuals—that occur more frequently in people with a particular trait such as fraternal twinning. If a genetic variant is consistently found more often in mothers of fraternal twins than in women who do not have twins, researchers can infer that the region of DNA containing that variant may influence the biological processes involved in twinning. Using this approach, researchers identified several genes linked to fraternal twinning²⁵. Some of these genes identified were found to be involved in regulating reproductive hormones and ovulation. A downside of this study is that it included only women of European ancestry²⁵. The researchers noted that studying populations in Africa, where twinning rates are highest, and Asia, where they are lowest, could reveal much more about the biology of twinning and female fertility²⁵.

In another genetic study, women with a higher polygenic score for natural fraternal twinning—meaning they carry more genetic variants associated with having twins—tended to conceive faster than other women²⁶. This suggests that the biological mechanisms involved in hyperovulation may also influence how efficiently the ovaries produce mature eggs. Researchers also found genetic correlations between fraternal twinning and several reproductive conditions, including anovulatory infertility, Polyendocrine Metabolic Ovarian Syndrome (PMOS), and endometriosis (Figure 4)²⁶. The overlap in genetic risk factors does not mean that the same single gene causes all of these conditions²⁷. Instead, the genetic correlations found suggest that some of the genetic factors involved in ovulation and ovarian function may overlap across these traits identified²⁷. Hormonal pathways involved in ovulation are complex and connected to many aspects of reproductive health⁷. At the same time, the genetic variants identified so far explain only a small portion of fraternal twinning, suggesting that many additional genes and biological factors involved in ovulation remain undiscovered. Most genetic studies have also focused primarily on European populations, despite some of the highest twinning rates occurring in West Africa. Studying populations with unusually high twin rates, such as those in southwestern Nigeria, could help researchers uncover new insights into both twinning and female fertility.

At first, Igbo-Ora might sound like a charming curiosity, a town where people see double. But this mystery matters. Twin pregnancies carry higher risks, including preterm birth and low birth weight¹⁹. Understanding why some populations experience more twin pregnancies can help to improve maternal care and prenatal planning. Studying Igbo-Ora may also teach us more about how ovulation is regulated, how genes and hormones interact, and why fertility varies across populations. Such studies may even improve fertility treatments in the future by helping doctors better understand hyperovulation.

Most human traits vary across populations. Igbo-Ora stands out as a rare and concentrated biological outlier, almost like nature’s own experiment in human reproduction. What looks like a small mystery in one town is a window into how the human body works. Scientists do not yet have a concrete answer, but Igbo-Ora gives researchers something valuable: a real-world example of how biology, genetics, and population patterns come together in unexpected ways. The streets of Igbo-Ora may be filled with matching faces and shared birthdays, but behind the celebration lies an important reminder: sometimes the most fascinating scientific questions are hiding in plain sight, in the middle of a town, where you cannot help but see double.

TL; DR

• Igbo-Ora in southwestern Nigeria is known as the “Twin Capital of the World”.
• Most of these twins are fraternal, caused by hyperovulation.
• Recent genetic studies suggest twinning may be linked to fertility and hormone regulation. Studying Igbo-Ora could reveal new insights into human reproduction.

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