Multiple births are more common than they were in the past, due to the advancing average age of mothers and the associated rise in assisted reproductive techniques, in particular the use of fertility drugs. Twins account for over 90 per cent of multiple births.
Types of twins
There are two types of twins – identical (monozygotic) and fraternal (dizygotic).
To form identical twins, one fertilised egg (ovum) splits and develops two babies with exactly the same genetic information. This differs from fraternal twins, where two eggs (ova) are fertilised by two sperm and produce two genetically unique children, who are no more alike than individual siblings born at different times. Twins are more or less equally likely to be female or male. Contrary to popular belief, the incidence of twins does not skip generations.
Predisposing factors of having twins
Some women are more likely than others to give birth to twins. The factors that are responsible for this include:
Advancing age of the mother – older women in their 30s and 40s produce more than one egg at a time due to oestrogen hormone stimulation than younger women.
Number of previous pregnancies – the greater the number of pregnancies a woman has already had, the higher her odds of conceiving twins.
Heredity – a woman is more likely to conceive fraternal twins if she is a fraternal twin, has already had fraternal twins, or has siblings who are fraternal twins.
Race – black African women have the highest incidence of twins, while Asian women have the lowest.
Assisted reproductive techniques – many procedures rely on stimulating the ovaries with fertility drugs to produce eggs and, often, several eggs are released per ovulation.
The normal length of gestation for a single baby is around 40 weeks. However, gestation for twins, either identical or fraternal, is usually around 38 weeks. This shorter time is due to the increased demands on the mother’s body, and the inability of the babies to receive all the nutrients they need in utero.
Since twins are usually premature, they are more likely to have lower birth weights. Prematurity is associated with increased risk of a number of disorders, including jaundice.
Giving birth to twins
Childbirth can give rise to complications when just one baby is present, so twins present extra potential for difficulties. It is advised that women carrying twins give birth in hospital, rather than at home. The babies can be delivered vaginally, but caesarean section delivery may be considered a better alternative in some circumstances.
How do identical twins happen?
Identical twins form when one egg has been fertilised by one sperm and the zygote splits into two. This happens at the very earliest stage of development, when the zygote is no more than a cluster of a few cells. Dividing this early in conception means that each baby has exactly the same genetic information as the other.
Identical twins can also occur when one a woman has undergone fertility assistance and has had one fertilised egg returned to her uterus. If this egg then splits into two, an identical twin pregnancy can result.
How do non-identical twins happen?
Non-identical twins are uniquely separate individuals who just happen to be gestating at the same time and place as each other. Non-identical twins form from two completely separate eggs which are fertilised by two completely separate sperm. These fraternal twins are no more alike than any other siblings in a family with the same biological mother and father.
Because identical twins share the same genes, they are always the same gender. But non-identical twins may be the same gender or the opposite because they only share 50% of the same genetic coding.
What causes the fertilised egg to separate into two?
The exact reason why this happens is still a mystery. Over the years there has been a lot of research and time invested into finding out why some zygotes split into two and others don’t. But the truth is that still do not know why.
In reality, the splitting of one fertilised egg is a malformation of the natural processes of conception.
When are identical twins actually formed?
Though all identical twins are formed very early in pregnancy, there is some variation in exactly how early the fertilised egg splits into two. This can happen at the two cell stage on Day 2, the early blastocyst stage on Day 4, or in the late blastocyst stage on Day 6. The stage when the egg splits into two will determine where the fertilised eggs will implant in the uterine wall. Once the embryo gets to the 8 cells stage the foetal DNA takes over. The stage at which the egg splits will also determine if the babies have their own amniotic membranes and placenta or if they share. Essentially, the earlier the egg splits, the more independent each twin will be by having their own amnion, chorion and placenta.
Diagnosis of twins’ pregnancy
It is difficult if not impossible for the mother to know if she is carrying twins’ pregnancy because the splitting of the zygote occurs so early in pregnancy. It can however be known through:
The intensity of pregnancy symptoms
The risks of having identical twins
Malnourishment: one will be well fed while other will not especially if the twins ahare same placenta.
Umbilical cord entanglement and compression if the twins share an amniotic sac.
Increased risk of pregnancy and birth complications.
Increased likelihood of prematurity and associated problems.
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