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Superfetation is a rare condition in which a woman becomes pregnant with twins but not at exactly the same time, with a few days difference in conception. This usually happens in women who are undergoing treatment to become pregnant, such as the use of ovulation inducers, which ends up delaying the interruption of ovulation.
Learn more about the different types of fertility treatments.
In a common pregnancy after conception, the woman's body prevents ovulation from happening again and therefore another egg cannot be fertilized. However, there may be some hormonal change that causes a woman to ovulate again, even though she has been pregnant for a few days, running the risk of being fertilized if she has unprotected intercourse, thus becoming pregnant with twins, when in reality should only be expecting 1 baby.

How to tell if twins are different ages
The only way to know that twins have different weeks to live is through an ultrasound, which indicates that one baby is less developed than the other. However, the fact that a woman is pregnant with twins in different stages of development does not always mean that there has been superfetation.
Initially the woman will not notice any difference and will discover that she is pregnant at the normal time, when she has symptoms such as dizziness, nausea, tender breasts or delay in menstruation. The doctor may suspect that it is a twin pregnancy when he finds that the levels of Beta HCG are very high and confirms that it is a twin pregnancy by ultrasound. And it is at this time that superfetation can be discovered. See normal levels of beta HCG.
Superfetation is a very rare condition and usually occurs in women who have become pregnant due to hormone treatment.
How can it happen
Pregnancy of twins of different ages can happen because the sperm remains alive inside the uterus for about 3 days. Assuming that the woman was ovulating and there was intimate contact, if 1 sperm manages to enter the egg, there will be conception and this indicates that she is pregnant with only 1 baby.
If for some reason, even after this conception, the woman presents another mature egg, if it is fertilized 2 or 3 days later by another sperm that may have come from the same sexual relationship or not, then she will be pregnant with the 2nd baby. In that case she will be pregnant with twins and they will be false twins, or fraternal twins, because each will have their own placenta.
How is childbirth
The most common is that the difference in days of conception of each baby is very small and therefore, it should not influence the time of birth. In any case, if the difference is large, with more than 4 weeks of difference between one baby and another, when the youngest one is ready to be born, delivery must be carried out, but this depends on several factors, because the older baby cannot spend more than 41 weeks in the womb.
Twins are usually delivered by cesarean section and need to stay in the hospital for a few days until they weigh more than 2 kg and are he althy to be discharged, which does not always happen at the same time.
Check out the precautions to be taken during pregnancy and childbirth.