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How the Twin Babies are Formed?

Sep 23, 2022

Know How Twins are Formed

When you are pregnant and carry two babies, they are called twin babies. If three babies, they are called triplets. High risk of complications like preterm delivery, preeclampsia, and improper fetal growth may happen with multiple babies. 

The two types of a twin pregnancy are-

  • A single fertilized egg divides into two eggs before it gets implanted in the uterine lining.
  • Fertilization happens with two eggs by two different sperm simultaneously.

These two different formations result in either identical or fraternal (non-identical) twins. 

The development of identical twins happens when a single egg is fertilized and splits into two different embryos. These divided embryos develop into identical monozygotic twins. Such babies look like each other and are of the same sex.

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Fraternal twins are formed from two separate eggs which are fertilized by two different sperm, and these embryos are dizygotic. Such babies may be of varying sex and not look identical to each other because they form from different eggs and sperm, and hence the genetic material may vary.

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Mostly, fraternal (Non-identical) twins may occur through fertility treatment because in fertility centers’ two fertilized embryos are placed into a woman’s womb to have a successful pregnancy.

Factors that Increase the Birth of Twins

The factors that may increase the feasibility of giving birth to twins are:

  • The age of a woman
  • Family history of twins
  • Through fertility treatments

The Age of a Woman

Research says women aged 30 years or above have more chances to conceive twins because women of this age produce excess follicle-stimulating hormone (FSH) more than younger women. Since FSH is one of the hormones that makes an egg mature for monthly ovulation, an excess FSH cause to release of more than one egg during the single menstrual cycle. After the eggs are released, if sperm fertilizes these eggs, a twin pregnancy may occur.

Family history

If a woman has a family history of twin births on the mother’s side than on the father’s side, she has a higher chance to get twin babies pregnancy naturally. In some cases, identical twins are not genetic-wise. However, fraternal (non-identical) twins may occur as per family history. 

Thinking how is that possible for only non-identical twins to have a genetic basis and the other does not?  Let us explain here revealing the biological structures.

As you know, the main characteristic feature of identical and fraternal twins is the release of the number of fertilized eggs. The division of a single fertilized egg results in identical twins, whereas fraternal twins occur from two different eggs. Since identical twins came from the same fertilized egg, they have identical DNA structures. This event may happen randomly and doesn’t impact the family's genes.

The same biological scenario does not happen for fraternal (non-identical) twins. Since fraternal twins occur from two independent eggs and two different sperms, their DNA structures are not similar. 

The release of two eggs in one cycle is called hyperovulation. The splitting of an embryo is not genetic, however, genes control ovulation, and each woman can have different ovulation gene terms. Some women have a specific ovulation gene history called alleles inherited from the parents to the child, which makes them undergo hyperovulation. This process increases the probability of having fraternal twins, and that's why the experts say fraternal twins run from the mother’s side family genes.

Fertility treatment

Recently, fertility treatments are the main factor in increasing the percentage of having twins. The technique of using some fertility drug treatment provokes the ovary of a woman to release more than one egg.

In vitro fertilization (IVF) treatment is carried out by fertilizing a woman’s egg with a donor’s sperm to produce an embryo. This artificially fertilized embryo is passed into the woman’s womb. The fertility centers may deposit more than one embryo to increase the success rate, whereas twins’ pregnancy may occur if both embryos develop successfully.


Related: How to Increase Egg Quality for Pregnancy?

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