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Charlotte Lozier Institute

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Charlotte Lozier Institute

Phone: 202-223-8073
Fax: 571-312-0544

2776 S. Arlington Mill Dr.
#803
Arlington, VA 22206

The
Voyage of Life

Twins

Dive Deeper
How often are twins born?

Twins are more common today than they were a generation ago, although most of the increase has been driven by fertility treatments rather than natural conception. In the United States, the twin birth rate rose by about 76% between 1980 and 2009, increasing from 18.8 to 33.3 babies per 1,000 births before declining to 30.1 by 2024.1

Even without fertility treatments, however, natural twinning rates vary dramatically around the world. The Yoruba people of West Africa have the world’s highest natural twinning rate, with 90-100 twin babies per 1,000 births,2 whereas parts of Southeast Asia have the lowest, with only 12-18 twin babies per 1,000 births.3 Scientists suspect that both genetics and diet contribute to these striking differences.

Identical twins, however, are a remarkable exception. They occur at a remarkably constant rate of about 3 to 4 pregnancies per 1,000 births worldwide, suggesting that the splitting of a single embryo is largely independent of genetics, environment, or diet.4

3D ultrasound image of 12 week old twins. Every time one twin moves, that twin may touch the other. Twins that touch each other provide some of the ultrasound evidence that the fetus moves when touched in the womb. (Image Credit: Science Source)
3D ultrasound image of 12 week old twins. Every time one twin moves, that twin may touch the other. Twins that touch each other provide some of the ultrasound evidence that the fetus moves when touched in the womb. (Image Credit: Science Source)
What about twinning in IVF?

Twin pregnancies are far more common after in vitro fertilization (IVF) because multiple embryos are often transferred to increase the chances of pregnancy. Researchers have also proposed that IVF may slightly increase the likelihood of identical twins by altering very early embryo development.5

How far apart can twins be born?

Most twins arrive only minutes apart. In one study, more than three-quarters were delivered within 15 minutes of each other.6 In extraordinarily rare cases, however, twins have been born months apart. The longest reported interval is 97 days, when twins delivered in Germany arrived more than three months apart.7

Identical twins come from one embryo that splits into two individual embryos. Fraternal twins come from two separate eggs that get [tooltip anchor="fertilized"]Sperm-egg fusion[/tooltip] separately. (Image Credit: Science Source)
Identical twins come from one embryo that splits into two individual embryos. Fraternal twins come from two separate eggs that get fertilized separately. (Image Credit: Science Source)
How do twins form?

Identical and non-identical twins form differently. Non-identical twins form when the woman ovulates more than one egg, and each egg gets fertilized, forming two separate zygotes. Alternatively, one zygote can become two people for up to 14 days after fertilization. This usually happens in one of two ways.  Early in development, the entire embryo may split, or just a few days later once the blastocyst has formed, the inner cell mass may split.8

When do twins form in the womb?

Scientists can infer when identical twins formed by observing the arrangement of the embryonic membranes such as the chorion and amnion. The amnion is a thin, tough sac surrounding the embryo and holding the amniotic fluid. The chorion surrounds the amnion, supplying it with nutrients. The chorion develops from the embryo’s cells and is responsible for absorbing nutrients and oxygen from the maternal blood in the placenta.

  • Identical twins that likely split around blastocyst hatching, before 5 days after conception: These twins have two different amniotic sacs, two chorions and two placentas. Roughly one-third of identical twins form this way.
  • Identical twins that likely split between days 4-8 after conception: These twins have two different amniotic sacs, but they share one chorion and one placenta. This can pose risks if the placenta and blood vessels start favoring one twin over the other. Roughly two-thirds of identical twins from between 4 and 8 days after conception.9
  • Identical twins that likely split from days 9-13 after conception: These twins share one amniotic sac, one chorion and one placenta. Only 1-2% of all identical twins split this late in development. These twins have the additional risk of tangling their umbilical cords.10

Despite risks during intrauterine development, twins that share a chorion did not significantly differ from twins who had their own chorion when tested for neurodevelopmental outcomes at two years old.11

Doctors can predict when identical twins separated by examining the number of placentas and amniotic sacs. (Image Credit: <a href="https://commons.wikimedia.org/wiki/File:Placentation.svg">Kevin Dufendach</a>)
Doctors can predict when identical twins separated by examining the number of placentas and amniotic sacs. (Image Credit: Kevin Dufendach)

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Fetal Phases

Week 2

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A close-up of an adult’s hands gently holding the tiny feet of a newborn baby, who is lying down and out of focus in the background. The image beautifully conveys tenderness, care, and the deep connection fostered during pregnancy. Dive Deeper

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DNA and Genetic Variation

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Back to The Voyage of Life
Sources
  1. ^ Martin, B. J. A., & Osterman, M. J. (2026). A Decade of Decline in Twin Childbearing in the United States, 2014–2024. https://www.cdc.gov/nchs/data/hestat/hestat117.htm
  2. ^ Nylander, P. P. S. (1971). Ethnic differneces in twinning rates in Nigeria. Journal of Biosocial Science, 3(2), 151-158. https://www.cambridge.org/core/journals/journal-of-biosocial-science/article/abs/ethnic-differneces-in-twinning-rates-in-nigeria/8A56E7CE6FDC6C73769F1C43A692C2E0
  3. ^ Smits, J., & Monden, C. (2011). Twinning across the developing world. PloS one, 6(9), e25239. https://pmc.ncbi.nlm.nih.gov/articles/PMC3182188/
  4. ^ Bortolus, R., Parazzini, F., Chatenoud, L., Benzi, G., Bianchi, M. M., & Marini, A. (1999). The epidemiology of multiple births. Human reproduction update, 5(2), 179-187. https://www.researchgate.net/profile/Liliane-Chatenoud/publication/12962764_The_epidemiology_of_multiple_births/links/004635391dc46b77b7000000/The-epidemiology-of-multiple-births.pdf
  5. ^ Matorras, R., Vendrell, A., Ferrando, M., & Larreategui, Z. (2023). Early spontaneous twinning recorded by time-lapse. Twin Research and Human Genetics, 26(3), 215-218. https://www.cambridge.org/core/journals/twin-research-and-human-genetics/article/early-spontaneous-twinning-recorded-by-timelapse/51AE12951DFA7BFDCD100DA0E575B82B
  6. ^ Stein, W., Misselwitz, B., & Schmidt, S. (2008). Twin‐to‐twin delivery time interval: influencing factors and effect on short‐term outcome of the second twin. Acta Obstetricia et Gynecologica Scandinavica, 87(3), 346-353. https://obgyn.onlinelibrary.wiley.com/doi/10.1080/00016340801934276
  7. ^
  8. ^ Condic, M. L. (2020). Untangling twinning: What science tells us about the nature of human embryos. University of Notre Dame Press. https://undpress.nd.edu/9780268107055/untangling-twinning/
  9. ^ Konno, H., Murakoshi, T., Miura, K., & Masuzaki, H. (2020). The incidence of dichorionic diamniotic twin pregnancy after single blastocyst embryo transfer and zygosity: 8 years of single-center experience. Twin Research and Human Genetics, 23(1), 51-54. https://doi.org/10.1017/thg.2020.5
  10. ^ Van Mieghem, T., Abbasi, N., Shinar, S., Keunen, J., Seaward, G., Windrim, R., & Ryan, G. (2022). Monochorionic monoamniotic twin pregnancies. American journal of obstetrics & gynecology MFM, 4(2), 100520. https://doi.org/10.1016/j.ajogmf.2021.100520
  11. ^ Hack, K. E., Koopman-Esseboom, C., Derks, J. B., Elias, S. G., de Kleine, M. J., Baerts, W., ... & A. Visser, G. H. (2009). Long-term neurodevelopmental outcome of monochorionic and matched dichorionic twins. PLoS One, 4(8), e6815. https://doi.org/10.1371/journal.pone.0006815
Learn more about Gestational Age

Credentialed scientists reviewed all the material on The Voyage of Life for accuracy. Ages are listed in gestational weeks, measured from the first day of the mother’s last menstrual period, unless otherwise noted. This standard medical practice differs from post-conception age, used by embryologists and medical textbooks cited here, which starts at conception, about two weeks later.

Medical art animation was used to provide a schematic representation of fetal development, with some features, including timing, not to scale. The animation was chosen as an ethically uncompromised guide to fetal development. There are many real human images at each developmental age showing the most accurate fetal forms throughout development. A concerted effort was made to use images and material in which the human embryo or fetus did not undergo any known harm.

Page last updated on August 6, 2026
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Sperm-egg fusion