Multiple births are more frequent than they have been in the past due to the rising average age of mothers and the resulting increase in assisted reproductive techniques, in particular, the use of fertility drugs.
Different types of twins account for more than 90% of multiple births. There are two types of twins – identical (monozygous) and fraternal (dizygous).
In order to form this type of twins (identical twins), one fertilized egg (ovum) separates and produces two babies with exactly the same genetic material.
This is different from fraternal twins, where two eggs (ova) are fertilized by two sperms and produce two genetically distinct children that are no more similar than individual siblings born at different times.
Twins are more or less likely to be male or female. Contrarily to common opinion, the occurrence of twins doesn’t skip generations.
Identical or ‘monozygotic’ twins
About one in three sets of twins is this type of twins. This is because the fertilized egg splits into two when it is only a small number of cells. The self-contained halves are then converted into two infants, with exactly the same genetic material.
Twins made from one egg and one sperm are considered identical or ‘monozygotic’ type of twins. The biological processes that lead the single fertilized egg to break into two remain a mystery.
About one-fifth of the identical twins are mirror images of each other, which means that the right side of the infant matches the left side of the twin.
Fraternal or ‘dizygotic’ twins
About two in three sets of twins are the fraternal type of twins. Two separate eggs (ova) are fertilized by two separate sperms, resulting in twins of a fraternal or ‘dizygotic’ type.
These babies are no longer than siblings born at different times. Babys may be either of the same sex or of different sexes, with the chances approximately equal for each.
The proposed ‘three-twin kind.’
Some researchers suggest that there could be a third type of twin, although the medical opinion is still divided. It is suggested that the egg should be divided into two, and then each half of the egg should be fertilized by a different sperm.
This hypothesis is an attempt to understand why certain fraternal twins look the same.
Factors that raise the likelihood of having any type of twins
Some women are likelier to have twins. Factors that raise the chances include the following:
Older Women – women in the 30s and 40s have higher levels of sex hormone oestrogen than younger women, which means that their ovaries are stimulated to produce more than one egg at a time.
- A number of pregnancies – the greater the number of pregnancies a woman has had, the greater her chances of conceiving twins.
- Heredity – a woman, is more likely to have fraternal twins if she is a fraternal twin, has twin brothers or siblings who are fraternal twins.
- Race: Black African women have the highest incidence of twins, while Asian women have the lowest incidence.
- Assisted reproductive techniques: several procedures depend on the stimulation of ovaries with fertility drugs to produce eggs, and sometimes many eggs are released per ovulation.
- Fertilization: Hormones secreted by the ovaries, and a tiny gland in the brain called the pituitary gland, regulate the menstrual cycle. The average period is approximately 28 days. After a menstrual cycle, the levels of the hormone oestrogen help to thicken the lining of the womb (endometrium) and release the egg from one of the ovaries (ovulation).
If the egg is fertilized on its path down the fallopian tube, it lodges in the thickened lining of the womb begins to differentiate and grows into an embryo.
Gestion for twins
The normal period of gestation for a single child is approximately 40 weeks. However, the gestation period for all types of twins, either identical or fraternal, typically takes around 38 weeks.
This shorter time is due to increased demands on the body of the mother and the inability of the baby to obtain all the nutrients required in the uterus.
Since twins are typically premature, they are more likely to have lower birth weights. Prematurity is associated with an elevated risk for a variety of diseases, including jaundice.
Childbirth can lead to problems when there is only one child, so twins have the extra potential for difficulty. It is recommended that women carrying twins give birth in a hospital rather than at home.
Babies may be offered vaginally, but in certain cases, delivery of the cesarean section may be considered a safer option.
Checking zygosity
It’s hard to tell what type of twins, whether twins are identical or fraternal at birth. Some identical twins can be born with individual sets of membranes, which can contribute to a misunderstanding that babies are fraternal.
One way to say the difference is to measure the DNA of the twins. Identical twins possess the same genetic information, while fraternal twins share only half of them.
The procedure can be performed with a sample of the cells of the cheek, obtained painlessly. Some checks include blood group checks.
Unusual events that may occur during twin pregnancy
Most twin pregnancy ends with the birth of two healthy infants. Occasionally, rare events occur during fertilization or at the early stage of the pregnancy that can lead to unique twins.
Mirror image twins
This is a subtype of identical twins that occurs when the egg splits 7 to 12 days after fertilization, instead of within the first week. At this time, the developing embryo had already grown to the left and to the right.
These twins are the same, but mirror copies of each other. For example, their hair will spiral in opposite directions, their teeth will begin to come in on opposite sides of their mouth, and one may be right-handed while the other is left-handed.
They might even cross their legs in opposite directions.
Conjoined twins
There are identical twins that are physically related to each other.
Some physicians claim it’s because of a fertilized egg that doesn’t break fully. This can happen when it divides 12 or more days after conception. Others claim it’s an egg that was totally divided, but then it was mixed back together.
The position of the fusion varies, but the chest or abdomen is the most common. The degree of fusion often varies, but the twins almost always share one or more vital organs.
Conjoined twins often die shortly after their birth. Those that live may be separated from each other, depending on where they are joined and which organs they share.
While they are together, these twins are two individuals who can think separately.
Monozygous (identical) twins boy/girl
In extremely rare cases, identical twins can be of different sexes. These twins begin with the same male twins. Like all males, they both have XY sex chromosomes instead of XX, as all females do.
Very soon after the egg divides into two, a genetic mutation causes one twin to lose the Y-sex chromosome, converting it to X0. The mutation is called Turner Syndrome.
Since there is only one X chromosome, the twin appears female but has developmental disabilities beginning at birth and fertility issues later in life. The other child is not affected.
Twins with different ages
Superfetation is when fertilization of a second egg occurs in a woman who’s already pregnant. This doesn’t often happen because women typically stop releasing eggs as soon as they get pregnant. When this occurs during the same menstrual cycle, it’s known as superfecundation.
Twins with different fathers
Heteropaternal superfecundation is when two ova released at different times in the same ovulation cycle are fertilized by different fathers. It is very rare in humans but common in animals.
Semi-identical twins
This form is the product of two separate sperms that fertilize a single egg. In order to survive, this egg must be divided into two, with each half having exactly the right number of chromosomes. Just two cases of semi-identical twins have been reported.
Parasitic twins
Parasitic twins are a type of conjoined twin, where a smaller twin depends on a larger twin. The smaller twin is not fully formed and does not have vital organs, such as a fully formed brain or heart.
A small twin can form anywhere on the other twin’s body and appear to be something like a small undetectable lump, a second non-functional head, or extra limbs attached to random body parts.
Parasitic twins subtypes include:
Fetus in a fetus
This is when a parasitic twin forms within the body of a larger twin rather than outside it.
The Acardiac twins
Twin to Twin Transfusion Syndrome can happen when one identical twin has too much blood supply, and the other gets too little through a shared placenta.
Acardiac twins have an extreme version of this, where the smaller twin is a torso with or without legs, the heart of which is missing or malformed.
Medical risks during different types of twin pregnancy
Pregnancies with multiple fetuses are considered high risk because they may have a higher chance of difficulties such as:
- Prematurity. Most infants in different types of twins pregnancy are born before 37 weeks of pregnancy.
- Postpartum haemorrhage. A woman can experience heavy bleeding after delivery because the placenta is large, and the uterus has stretched too much.
- Gestational diabetes. A mother’s blood sugar can get high during pregnancy.
- Placenta previa. The placenta is low and covers the cervix.
- Placental abruption. The placenta tears away from the uterine wall.
- Placenta accreta. Blood vessels and other parts of the placenta grow too deeply into the uterine wall.
- Low birth weight. Birth weight is less than 5 pounds, 8 ounces.
- Gestational hypertension. A mother has high blood pressure while pregnant.
