Twins Conceived at Different Times

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Definition of Superfetation

Can you get pregnant if you are already pregnant? And would the result be twins? The term used to describe this situation is superfetation.  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 occurs when ova from two separate menstrual cycles are released, fertilized, and implant in the uterus.

Normally, once a woman is impregnated, physical and hormonal effects would make this impossible. First, her hormones act to halt the process of ovulation and prevent the release of another egg from her ovaries. 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 twins, where multiple ova are expelled in a single cycle. When more than one ova is fertilized and implants in the uterus, the result is dizygotic twins. Though two fetuses develop simultaneously in superfetation, they differ in maturity, having been conceived days or even weeks apart. Although superfetation is observed in animal reproduction, it is exceedingly rare in humans. Only a few cases are documented in medical literature, and it can be difficult to distinguish a case of twins with growth discordance and a true case of superfetation.


Examples of Superfetation

In 2009, an Arkansas woman reportedly became pregnant with two babies due to superfetation. Ultrasound revealed that Julia Grovenburg 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.

a thirty-two 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. 


Blickstein, I. and Keith, L. Multiple Pregnancy: Epidemiology, Gestation, and Perinatal Outcome. CRC Press, 2005. Print.

Cassimally, K. "Superfetation: Pregnant while already pregnant." Scientific American. Accessed January 7, 2016.

Jick, B. "Superfetation (or “Getting Pregnant While Already Pregnant!”)." Pregnancy Corner. Accessed January 7, 2016.

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