Superfetation: Twins Conceived at Different Times

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Can you get pregnant if you are already pregnant and would the result be twins? The term used to describe this situation is superfetation and the babies are considered an unusual type of twinning.

Superfetation is the formation of a fetus while another fetus is already present in the uterus. Essentially, it describes a situation where a woman becomes pregnant when she is already pregnant. It is believed that this is a very rare event and only a few cases have been reported and verified.


Superfetation occurs when ova from two separate menstrual cycles are released, fertilized, and then implant in the uterus. Normally, once a woman is impregnated, physical and hormonal effects would make this impossible.

Hormones are the first barrier in a normal pregnancy. They act to halt the process of ovulation and prevent the release of another egg from her ovaries.

The uterine lining also changes after one embryo has implanted, making further implantation difficult. Another physical effect of pregnancy, the mucus plug, makes it difficult for sperm to breach the cervix during intercourse.


Superfetation differs from the conception of typical twins, where multiple ova are released in a single cycle. This can happen naturally or be stimulated with fertility drugs. When more than one ovum is fertilized and implants in the uterus, the result is dizygotic (fraternal) twins, triplets, or other multiples.

Although two fetuses develop simultaneously in superfetation, they differ in maturity, having been conceived days or even weeks apart. Superfetation is observed in animal reproduction, but it is exceedingly rare in humans. Only a few cases are documented in medical literature.

Superfetation is suspected only when the twins are of different sizes and at different stages of development. It is typically noticed during a routine checkup on the ultrasound. However, it can be hard to distinguish whether this is a true case of superfetation or due to other factors.

Superfetation is different from superfecundation. This similar-sounding term describes twins with different fathers. In superfecundation, the timing is different, with conception occurring on the same day or within a few days.


In 2016, Australian Kate Hill became pregnant with two babies and had only had unprotected sex once. She had been receiving hormone treatments for polycystic ovary syndrome (PCOS), a condition that decreases ovulation.

Doctors discovered that her twins were conceived 10 days apart and the fertilization was the result of just one ejaculation of sperm. What makes this case even more unusual is that it's generally accepted that sperm have a lifespan of around 5 days.

Julia Grovenburg is an Arkansas woman who reportedly became pregnant with two babies due to superfetation in 2009. An ultrasound revealed that she was pregnant with two babies conceived about two and a half weeks apart.

A case of superfetation in a triplet pregnancy was described in The Journal of Pediatrics in 2005. In this case, a 32-year-old woman became pregnant with the aid of fertility treatments. After two transferred embryos produced a twin pregnancy, a third fetus was discovered several months into the pregnancy and determined to be developing about three weeks behind the other two babies.


If the two fetuses are at very different lengths of gestation, there is a risk for the younger one to be delivered prematurely. As there is a higher rate of twins being born preterm at less than 37 weeks, this increases the risk for the younger twin.

Most of the verified cases have been with twins that are only 10 days to three weeks apart, and the reported cases were usually safely delivered.

A Word From Verywell

Verified cases of human superfetation are rare. While it may be an interesting phenomenon, it isn't one to worry about happening in your pregnancy.

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  1. Harrison A, Valenzuela A, Gardiner J, Sargent M, Chessex P. Superfetation as a cause of growth discordance in a multiple pregnancyThe Journal of Pediatrics. 2005;147(2):254-255. doi:10.1016/j.jpeds.2005.04.038

Additional Reading
  • Harrison A, ValenzuelaA, Gardiner J, Sargent M, Chessex P. Superfetation as a Cause of Growth Discordance in a Multiple Pregnancy. The Journal of Pediatrics. 2005;147(2):254–255. doi: 10.1016/j.jpeds.2005.04.038.

  • Tarín JJ, García-Pérez MA, Hermenegildo C, Cano A. Unpredicted Ovulations and Conceptions During Early Pregnancy: An Explanatory Mechanism of Human Superfetation. Reproduction, Fertility, and Development. 2013;25(7):1012. doi: 10.1071/rd12238.

  • BBC News. Australian Woman Gets Pregnant Twice in 10 Days. 2016.
  • Cox L, Arkansas Pregnant Woman Is Pregnant Again. ABC News. 2009.