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Superfetation is defined as another conception during an ongoing pregnancy. It refers to pregnancy with two fetuses of different gestational age; meaning ovulation, fertilisation and implantation of a second or additional embryo(s) days or weeks after the first one.
Superfetation is an extremely rare case of twin pregnancy and is also different from the process of multiple gestation or twinning. In the prior, both the fetuses are at different developmental stages as they get implanted at gestational age. However, in the latter, both the fetuses have the same delivery or due dates as they get implanted simultaneously.
This type of pregnant is mainly found in rodents and livestock, but in humans, explanations on such type of pregnancies are controversial due to missing of artefacts. It is also seen as an evolved reproductive strategy in mammals (such as humans). 
In this article, we will discuss details on superfetation or the concept of getting pregnant while pregnant. Take a look.
How Fertilisation Occurs?
Fertilisation is the fusion of the sperm with the egg. During intercourse, millions of sperm enter the vagina, out of which, a large number of them die in the acidic environment of the vagina while some in their journey towards the uterus.
Sperms that are able to enter the uterus, are moved towards the egg by contraction of this reproductive organ. The egg then gets fertilised with the sperm in the oviduct.
The fusion of one sperm out of millions is governed by the egg cortical reaction. When the egg and sperm get fused, the release of calcium ions is triggered that says the egg cortical granules to release their contents outside of the cell to prevent the entry of other sperms as well as create a protective layer around the fertilised egg.  This helps prevent another fertilisation in the egg.
Why Superfetation Is Unlikely To Occur?
Superfetation is unlikely is occur because of the following reasons:
- As soon as the fertilisation occurs, the body stops further ovulation or say, the release of the second egg, unless both are releases at the same time such as in case of fraternal twins.
- The release of hormones after fertilisation is likely to prevent further ovulation.
- Suppose the second egg gets fertilised weeks after the first one. This suggests that the sperm might have lived for around a week in the vagina, which is quite impossible as they can only live in a female body for up to five days. Even, if you have had sexual intercourse after the first fertilisation, the condition is rare to occur.
- The formation of mucus plug around the fertilised egg prevents the entry of other sperms into the cervix.
- After implantation of one embryo, the uterine lining changes, making the environment unfavourable for the implantation of another embryo.
How Does Superfetation Occur?
The main reason for the occurrence of superfetation is fertility treatments. Women who are under some kind of hormonal therapies such as in polycystic ovary syndrome, the overall procedure can sometimes mess with the ovulation and fertilisation.
A case report of a 30-year-old says talks about superfetation while she was undergoing fertility treatment through intrauterine insemination. The dual fertilisation took place when the artificially fertilised egg starts developing in the womb and then, another egg got fertilised through normal process i.e sexual intercourse, days or week later after the first fertilisation. 
Another case study talks about the diagnosis of dichorionic-diamniotic pregnancy in the first trimester, a twin pregnancy in which each embryo has its own chorionic and amniotic sacs. 
A triplet pregnancy case report also talks about superfetation in which a 32-year-old woman, who was undergoing fertility treatments, got twins embryos transferred to her uterus and then, was diagnosed with the third embryo three weeks later. 
Another cause of superfetation is the use of clomiphene citrate, a type of medication used to treat infertility problems in women by increasing the production of follicle-stimulating hormone and luteinizing hormone, the hormones that initiate ovulation. 
Complications Of Superfetation
The main complication related to superfetation is preterm birth of the second baby due to difference in the implantation period. This may give rise to problems such as low birth weight, brain haemorrhage or cognitive problems. Another complication is growth discordance as the two fetuses are of different sizes. 
Superfetation can also increase the maternal risk of high blood pressure and gestational diabetes.
How To Prevent Superfetation
Though superfetation is extremely rare, its chances can be lowered by not having sex during the first trimester of pregnancy.  Also, you should make sure that you are not pregnant before starting with fertility treatments such as IVF, including times when you have not taken a daily dose of injections.
Studies suggest that superfetation can be due to some rarely possible machanism, including genetic abnormalities. Early detection and treatment of superfetation can help with a healthy pregnancy by preventing severe symptoms such as abdominal pain and complications such as preterm birth.