As one literature report begins,
The topic is superfetation – the phenomenon of becoming pregnant whilst already pregnant.
It happens in humans (we think). And hares[2]. It’s been documented in badgers, mink, panthers, buffalo, wallaby, rats, mice, rabbits, horses, sheep, kangaroos, sugar gliders and cats. But much of the evidence is dubious, and remains controversial. The only agreed incidents seem to be documented in fish that carry their young – the poecilid and zenarchopterid.
Superfetation shouldn’t happen. Once conception has occurred, hormones are released that prevent further ovulation, and a mucus plug blocks up the womb.
But does it happen from time to time anyway?
Many critics argue that these superfetation-like pregnancies have other origins.
Because it’s rare, we don’t know any symptoms to look out for. It’s usually diagnosed when “discordantly developed young” are seen with separate amniotic sacs that can’t be explained any other way, but there have never been discordantly developed young documented that are more than a few weeks apart in growth. This means it’s quite possible the discordant growth is caused by something else not yet accounted for. Doctors have suggested placental insufficiency (when the placenta can’t support both foetuses so concentrates on one), twin-to-twin transfusion (when blood is unevenly shared between twins), or embryonic diapause (developmental arrest of one or more foetuses, which is driven by hormones and happens when environmental conditions are adverse)[3].
Other scientists think superfetation may be a cool reproductive strategy to maximise their number of offspring. If it is, it opens up more questions than it answers, including why it’s so rare and what provokes it when it happens, how it evolved, how the endocrine system regulates it, and how the maternal and foetal microbiome and immune systems behave.
These scientists have attempted to classify superfetation into three groups[1]:
1. Superfertilisation (or superfecundation) describes the fertilisation of two or more ova by two different males. It happens when there are two or more eggs released together in one cycle, and two sexual acts within the fertility window. The result is twins babies with different biological fathers.
2. Superconception is what scientists think happens in “permanently pregnant” hares[2]. Towards the end of pregnancy, the female ovulates and mates. Researchers have shown this ovulation using high-resolution ultrasound imaging to show fresh corpora lutea (cysts that form when eggs exit the ovaries). After giving birth, the egg then implants in the now-available womb. The pregnancies may or may not technically overlap, depending on how you classify them, but the phenomenon allows the hares to produce up to 35.4% more offspring in a breeding season.
3. In superfetation proper, a second ovulation cycle, or several subsequent ovulation cycles, happens during pregnancy, and a second foetus or litter is conceived. This produces two babies or litters, the younger of which is at risk of premature birth. This is most likely to happen when the female has two uteruses, or if implantation of the first embryo is delayed, so cycle-suppressing hCG hormones come too late. In humans, superfetation proper generally happens after fertility treatment, when a woman’s natural cycle isn’t sufficiently suppressed[4].
Scientists may not be sure whether superfetation is real or not, but it’s still a mildly worrying thought. Nevertheless, it’s not recommended to take birth control whilst pregnant, and luckily, your chances are pretty slim!
There are many unknowns when it comes to pregnancy, and over the next few months, I’ll be exploring more of them with you. Look out for my next blog post, which will be about baby tastes and food preferences.
To read our full article on the things we don't know about pregnancy, check out our site. This article will be updated as we add posts.
References“Here we review the scant and at times ancient literature on this poorly understood topic.”[1]
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| European hare. By Jean-Jacques Boujot via Wikipedia Commons. |
Superfetation shouldn’t happen. Once conception has occurred, hormones are released that prevent further ovulation, and a mucus plug blocks up the womb.
But does it happen from time to time anyway?
Many critics argue that these superfetation-like pregnancies have other origins.
Because it’s rare, we don’t know any symptoms to look out for. It’s usually diagnosed when “discordantly developed young” are seen with separate amniotic sacs that can’t be explained any other way, but there have never been discordantly developed young documented that are more than a few weeks apart in growth. This means it’s quite possible the discordant growth is caused by something else not yet accounted for. Doctors have suggested placental insufficiency (when the placenta can’t support both foetuses so concentrates on one), twin-to-twin transfusion (when blood is unevenly shared between twins), or embryonic diapause (developmental arrest of one or more foetuses, which is driven by hormones and happens when environmental conditions are adverse)[3].
Other scientists think superfetation may be a cool reproductive strategy to maximise their number of offspring. If it is, it opens up more questions than it answers, including why it’s so rare and what provokes it when it happens, how it evolved, how the endocrine system regulates it, and how the maternal and foetal microbiome and immune systems behave.
 |
| Zenarchopterid fish. Neale Monks via Wikipedia Commons. |
1. Superfertilisation (or superfecundation) describes the fertilisation of two or more ova by two different males. It happens when there are two or more eggs released together in one cycle, and two sexual acts within the fertility window. The result is twins babies with different biological fathers.
 |
| Leverets. via Wikipedia Commons. |
3. In superfetation proper, a second ovulation cycle, or several subsequent ovulation cycles, happens during pregnancy, and a second foetus or litter is conceived. This produces two babies or litters, the younger of which is at risk of premature birth. This is most likely to happen when the female has two uteruses, or if implantation of the first embryo is delayed, so cycle-suppressing hCG hormones come too late. In humans, superfetation proper generally happens after fertility treatment, when a woman’s natural cycle isn’t sufficiently suppressed[4].
 |
| Twins. MultipleParent via Wikipedia Commons. |
Scientists may not be sure whether superfetation is real or not, but it’s still a mildly worrying thought. Nevertheless, it’s not recommended to take birth control whilst pregnant, and luckily, your chances are pretty slim!
There are many unknowns when it comes to pregnancy, and over the next few months, I’ll be exploring more of them with you. Look out for my next blog post, which will be about baby tastes and food preferences.
To read our full article on the things we don't know about pregnancy, check out our site. This article will be updated as we add posts.
why don't all references have links?
[1] Roellig, K., Menzies, B. R., Hildebrandt, T. B., & Goeritz, F. (2011). The concept of superfetation: a critical review on a ‘myth’in mammalian reproduction. Biological Reviews, 86(1), 77-95. doi: 10.1111/j.1469-185X.2010.00135.x.
[2] Roellig, K. et al. Superconception in mammalian pregnancy can be detected and increases reproductive output per breeding season. Nat. Commun. 1:78 doi: 10.1038/ncomms1079 (2010)."
[3] Chapter 4 - Abnormal development of the conceptus and its consequences, pp. 119-143. In: Noakes, D.E, Parkinson, T.J., England, C.W., & Arthur, G.H. (2001) Arthur's Veterinary Reproduction and Obstetrics 8th Edition. Elsevier Ltd. ISBN: 978-0-7020-2556-3. doi: 10.1016/B978-070202556-3.50008-6.
[4] Harrison, A., Valenzuela, A., Gardner, J., Sargent, M. & Chessex, P. Superfetation as a cause of growth discordance in a multiple pregnancy. The Journal of Pediatrics 147:2, 254-255 (2005). doi: 10.1016/j.jpeds.2005.04.038.
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