Some weeks back, I came across a case report published in 1999 in the journal Ultrasound in Obstetrics and Gynecology [1]. Information technology presented a twin pregnancy wherein one of the fetuses seemed to exist at a younger developmental phase in its mother's womb compared to its sibling.

It wasn't the starting time time that I had stumbled on a report which detailed a discordance in the growth of a fetus in a twin pregnancy. In fact, cases of growth discordance in multiple pregnancies are not uncommon and may be due to a number of issues such equally the placenta's inability to adequately support the development of more than one fetus, twin-to-twin transfusion syndrome whereby blood is disproportionately transferred from 1 twin to the other, or other disorders which may every bit easily occur in unmarried pregnancies (congenital infection, aneuploidy, etc). However, the 1999 case study observed that the twin pregnancy was uneventful and resulted in the delivery of live healthy babies. It concluded that a miracle known as superfetation had occurred.

Superfetation is the onset of a subsequent pregnancy during an ongoing pregnancy; a new pregnancy during an initial pregnancy. It happens when at that place is fertilization of an ovule by a sperm jail cell while a conceptus is already present in the womb. Intriguing, isn't information technology? I thought then and dug deeper in the literature to realize that superfetation had really been extensively reported in animals [2].

The phenomenon of superfetation in animals has long been speculated as far back as fourth century B.C by Aristotle. Aristotle came to this thought when he observed that hares often gave birth to litters which comprised sets of healthy and imperfect offsprings. He deduced that the imperfect offsprings were younger ones who had spent less time in their female parent'southward womb—a straight consequence of superfetation. In effect, Aristotle was supposing that the younger offsprings were from a litter which was the upshot of the 2d pregnancy.

The annoy, American mink, panther, buffalo and swamp wallaby are a few examples of mammals in which superfetation is well documented. Last year, a paper published in Nature Communications reported that superfetation oft occurred in the European dark-brown hare, fifty-fifty postulating that it may actually be an evolutionary accommodation [3]. For this reason, it is likely that many more mammals may be accustomed to superfetation.

While at that place is evidence that superfetation may indeed exist part of the reproductive processes of sure animals, in humans, it is in all likelihood a rare reproductive aberration [2]. In other words, it probably occurs by blow. Because for superfetation to occur, a number of things must happen—things that a woman'south reproductive wheel is intrinsically programmed to preclude.

First, for any pregnancy to occur, an ovule must be present. And for an ovule to be present, ovulation must have taken place. Therefore, for superfetation to be possible, ovulation must be triggered during an ongoing pregnancy. And this is a major stumbling block correct there. While ovulation happens every month in a fertile woman, information technology is normally impeded during pregnancy. During the early stages of pregnancy, this is due to the corpus luteum, the surrounding tissue from which the ovule is shed during ovulation. It remains for a couple of days and releases hormones which ultimately prevent farther ovulation. If implantation of a conceptus then occurs, the corpus luteum stays for longer still, actively secreting more hormones. Eventually this endocrine role is taken over by the placenta. Therefore, strictly speaking, ovulation can only occur when neither the corpus luteum nor the placenta is present.

Pregnancy also requires the fertilization of the ovule with a sperm cell. This ways that semen from the male must exist able to attain the ovule in the female person's oviduct. When a adult female is significant however, a mucus plug is formed in the cervix, blocking the passage of sperm. This mechanism prevents semen from passing through the pregnant uterus, let alone allowing it to reach the oviduct. The machinery is an of import measure against microbes contained in semen which can cause crabs diseases to the fetus. It also prevents the possibility of superfetation.

Finally, a feasible pregnancy likewise requires proper implantation to occur. And for superfetation to occur, the conceptus needs to implant in a pregnant uterus. Implantation is only possible in a frail and specific environment controlled by different hormones though. Under normal conditions, the appropriate uterine environment is catered for in a woman who is ovulating rather than in one who is pregnant. In add-on, space is also an issue in the crowded already-occupied uterus!

In social club for superfetation to occur in humans, therefore, it would appear that three seemingly incommunicable things need to happen: ovulation must have identify during an ongoing pregnancy, semen must somehow find its manner through the blocked neck to the oviduct, via the occupied uterus and finally, the conceptus has to successfully implant itself in an unsuspecting already-occupied uterus. The odds of all three of them happening are without dubiety extremely small as reflected by the very occasional reports of superfetation in humans in the medical literature.

It should not exist surprising and so, that reports of superfetation in humans are more commonly associated with women who accept undergone fertility treatments. At least this is what I found when during my excavation. A example report published in 2005 in the The Journal of Pediatrics, for case, describe how a 32-year-one-time adult female who had undergone embryo transfer later on became pregnant with twins and one boosted growth-discordant fetus [four]. Initially, ii out of the iii embryos transferred to the woman's uterus implanted themselves and a viable twin pregnancy was pronounced. The 3rd embryo did not develop into a fetus. However, a triple pregnancy was discovered around five months afterwards, when a tertiary smaller fetus was seen past ultrasound. This fetus was deduced to be around iii weeks younger than its siblings. This led to the decision that in that location had been a subsequent fertilization and implantation post-obit the successful double implantation from artificial insemination.

During fertility treatments, information technology is hoped that a woman'southward reproductive cycle will come up to terms with events that information technology normally would not take to go through. Taking the above case to illustrate, 3 embryos were artificially transferred into the woman's uterus. The norm nevertheless, is one ovule per bike and thus one embryo for implantation—not iii! But when the human trunk is faced with the unusual aspects of pregnancy brought almost by artificial reproductive technologies, some mechanisms are more prone to get astray. For example, it is documented that a normal ovulatory response can result in post-obit cycles of controlled ovarian hyperstimulation even if an early pregnancy is present [5].

While to me the miracle of superfetation seemed and so astonishing that I decided to write a blog post near it, I'm not so sure if superfetation is looked upon with the aforementioned awe by the girls. Let me know in the comments.

References:

[1] Tuppen, Yard.D., Fairs, C., de Chazal, R.C. & Konje, J.C. Spontaneous superfetation diagnosed in the first trimester with successful outcome. Ultrasound in Obstetrics and Gynecology xiv, 219-221 (1999).

[two] Roellig, K., Menzies, B.R., Hildebrandt, T.B. & Goeritz, F. The concept of superfetation: a critical review on a 'myth' in mammalian reproduction. Biological Reviews

[3] Roellig, Chiliad., Goeritz, F., Fickel, J., Hermes, R., Hofer, H. & Hilderbrandt, T.B. Superfetation in mammalian pregnancy can exist detected and increases reproductive output per convenance flavour. Nature Communications 1:78 (2010).

[4] Harrison, A., Valenzuela, A., Gardner, J., Sargent, Grand. & Chessex, P. Superfetation as a crusade of growth discordance in a multiple pregnancy. The Journal of Pediatrics 147:2, 254-255 (2005).

[5] Dmowski, P.Due west., DeOria, L. & Rana, Northward. Embryo implantation during menstruation in the absence of acceptable estradiol and progesterone back up, with subsequent normal response to ovulation induction and superfetation. Fertility and Sterility 68:three, 538-541 (1997).

Image credits: Photograph 1: Ovary well-nigh to release an ovule (credit: Maksim from Wikipedia); Photo 2: Uterus at third to 4th month of pregnancy (credit: Magnus Manske from Wikipedia)

Virtually the Writer: Khalil A. Cassimally is a science pupil by day and scientific discipline blogger and community weblog director at Nature Educational activity by nighttime. He writes about the scientific discipline that gets him all excited. He also tweets @notscientific.

The views expressed are those of the author and are non necessarily those of Scientific American.

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