They include:. Sometimes, more than one egg is released during ovulation. If two eggs are fertilized during sexual intercourse and both successfully implant in the uterus, the result is a multiple pregnancy. If more than two eggs are released, fertilized, and implant, the result is multizygotic multiples, higher-order multiples such as triplets 3 , quadruplets 4 , quintuplets 5 , sextuplets 6 , septuplets 7 , octuplets 8 , or even more, although no multiples beyond octuplets have ever been known to survive.
Dizygotic or fraternal twins form in the same way that all humans do, by the union of sperm and egg. The explanation for dizygotic twinning lies in the cause of hyperovulation, which is the release of more than one egg in a cycle of ovulation. There are numerous reasons for hyperovulation and any one or combination of factors can be attributed as the cause for fraternal twins. Hormones control the process of ovulation.
Usually, they signal the body to release one egg in a cycle, but sometimes they trigger the release of two or more eggs. Some of the factors that may have an effect on hormones and influence this process are:.
Some other factors are thought to generate hyperovulation in women and cause twins, such as:. The causes of identical twinning are much vaguer. No clear explanation is offered by science. No specific theory has been confirmed as to why a fertilized egg splits and develops into two embryos. Data about monozygotic twins in populations indicates that the rate generally remains stable across populations and time periods.
As technology improves, scientists are getting closer to finding answers. One study used specialized computer software to capture photos of embryos developing and found that the embryo basically collapses, splitting the progenitor cells in half and dividing them into two sets of genetic material that form as two separate fetuses.
No genetic link has been identified. Some theories have been proposed but not confirmed. These include:. To increase the likelihood of success, the healthcare professional may place more than one embryo into the womb.
Twins can occur if both embryos implant and develop successfully. The reason for this is that women of this age are more likely than younger women to release more than one egg during their reproductive cycle. If sperm fertilizes two separate eggs, a twin pregnancy can occur. The ASRM report that nonidentical twins are slightly more common in taller or heavier women than they are in small women.
The reasons for this are not clear, but it may be due to better nutrition. Women with a higher body weight may have more resources available for a developing fetus. In the U. There are many unproven claims about how to increase the likelihood of conceiving twins. Some people advise following specific diets or using certain alternative therapies, but there is no scientific evidence to support these methods.
Fertility treatments, particularly IVF and ovary stimulants, do increase the chances of having twins. However, a twin pregnancy is riskier for both the woman and the developing fetuses.
For this reason, some fertility clinics advise against implanting multiple embryos during IVF treatment. The CDC recommend that younger women who are having their first fertility treatment select just one embryo for transfer to their womb. As the success of fertility treatment is improving, there is often less need to transfer more than one embryo. Transferring just one or two embryos reduces the likelihood of a multiple pregnancy. Although certain factors increase the chance of having twins, there is no way to improve the odds of having twins naturally.
A woman will usually find out that she is pregnant with twins from an ultrasound early on in the pregnancy. Some symptoms may indicate a twin pregnancy, including more intense morning sickness and very rapid weight gain. Occasionally they look quite similar. Very rarely, fraternal twins share a placenta. These types of twins are called chimaeric twins.
Sometimes a fertilised egg splits within a few days of conception to produce genetically identical twins. Because these twins come from one zygote, they are also known as monozygotic. Identical twins are the same sex. About one-third of identical twins split soon after fertilisation and form completely separate twins.
Like fraternal twins, these twins have separate placentas. The other two-thirds split after they attach to the wall of the womb. As a result, they share a placenta. The technical name for this is monochorionic. In a very small number of identical twins, splitting might happen even later. In this case, both twins share an inner sac , called the amnion, in addition to sharing a placenta.
The technical name for this is monoamniotic twins. The site is secure. If you are pregnant with more than one baby, you are far from alone. In the past two decades, the number of multiple births has climbed way up in the United States.
In , , twin babies and 6, triplet babies were born in the United States. In , there were only 69, twin and 1, triplet births. Why the increase? For one, more women are having babies after age Women in their 30s are more likely than younger women to conceive more than one baby naturally. Another reason is that more women are using fertility treatments to help them conceive.
Identical twins occur when a single fertilized egg splits into two.
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