This occurs because the fertilised egg divides in two while it is still a tiny collection of cells. The self-contained halves then develop into two babies, with exactly the same genetic information. The biological mechanisms that prompt the single fertilised egg to split in two remain a mystery. Approximately one quarter of identical twins are mirror images of each other, which means the right side of one child matches the left side of their twin. These babies will be no more alike than siblings born at separate times.
The babies can be either the same sex or different sexes, with the odds roughly equal for each. It is proposed that the egg splits in two, and each half is then fertilised by a different sperm. This theory is an attempt to explain why some fraternal twins look identical.
Gestation for twins 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. 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. Zygosity testing It is difficult to tell if twins are identical or fraternal at birth. Some identical twins may be born with individual sets of membranes, which may lead to the mistaken assumption that the babies are fraternal. One way to tell the difference is to have the twins DNA-tested.
Identical twins share the same genetic information, while fraternal twins share around half. The test can be done with a sample of cheek cells, collected painlessly. Other tests include blood group examinations. Where to get help Your doctor Paediatrician Maternity hospital Things to remember Twins account for over 90 per cent of multiple births. In a study published today Tuesday in Human Reproduction , one of the world's leading reproductive medicine journals, researchers have investigated one of the reasons why this happens and have, for the first time, been able to calculate that the proportion of multiple pregnancies after SET is 1.
These results come from the largest study to investigate zygotic splitting after SET -- it analysed , SET cycles -- and it highlights factors that could increase the risk. These include using frozen thawed embryos for SET, maturing the fertilised egg blastocyst in the laboratory for five or six days before SET, and assisted hatching, in which a small hole is created in the layer of proteins surrounding the embryo the zona pellucida to help the embryo hatch out and attach itself to the wall of the woman's womb.
One of the authors of the study, Dr Keiji Kuroda, of the Sugiyama Clinic Shinjuku and Juntendo University Faculty of Medicine in Japan, said: "As a result of our findings, clinicians may want to consider whether they should counsel couples about the small increase in the risk of multiple pregnancies as a result of zygotic splitting associated with some embryo manipulations.
A zygote is the fertilised egg cell that results from a man's sperm fertilising a woman's egg, and it contains all the genetic information from both parents to form a new individual. It soon starts to divide and subdivide into many more cells called blastomeres, which eventually form the embryo.
Zygotic splitting occurs between days two and six when the zygote divides, usually into two, and each zygote then goes on to develop into an embryo, leading to identical twins or triplets if it divides into three.
These are known as "monozygotic" twins or triplets. It can be difficult to identify whether a multiple pregnancy has occurred after true zygotic splitting or as a result of SET combined with sexual intercourse that results in another egg being fertilised at the same time. The only way to be sure is to use ultrasound to see whether there are one or more gestational sacs and to detect the fetus or fetuses via their heartbeats.
For this study, the researchers identified pregnancies arising from true zygotic splitting as those in which the number of foetuses exceeded the number of gestational sacs. After SET using fresh or frozen and then thawed embryos, there were nearly , clinical pregnancies Such a low incidence of MZT should encourage good prognosis patients to proceed with blastocyst transfers to maximize their pregnancy rates and decrease the number of transferred embryos. To view the article, please click here : Incidence of monozygotic twins in blastocyst and cleavage stage assisted reproductive technology cycles.
Monozygotic Twins. Splitting of the embryo may occur at any time during the first 2 weeks after fertilization, resulting in several forms of monozygotic twins. In about one third of the MZT cases, the splitting occurs within 72 hours of fertilization and the placenta will be dichorionic-diamniotic i.
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