Fraternal twins, which are not identical twins, are the most common type of twins.
Two-thirds of all twins in the world are fraternal. Uniform around the world; about 3 in 1,000. Only one-third of all twins in the world are identical.
You've probably heard of identical and fraternal twins, but a report released this week says there's a third kind -- sesquizygous twins or "semi-identical." Researchers say they share anywhere from 50 to 100% of their genomes.
Barring pregnancies that result from assisted reproductive technology, dizygotic twins are far more common than monozygotic twins and account for 70 percent of all twin gestations.
About 23 in every 1000 births are fraternal, a much higher rate than the three or four out of 1000 births for identical twins.
Everyone has the same chance of having identical twins: about 1 in 250. Identical twins do not run in families. But there are some factors that make having non-identical twins more likely: non-identical twins are more common in some ethnic groups, with the highest rate among Nigerians and the lowest among Japanese.
The chances of having identical twins is relatively rare. For every 1,000 births, identical twins happen every 3rd or 4th. Although, the use of fertility treatments will increase the risk of having identical twins. Identical twins actually make up 0.95% of pregnancies that were conceived with treatment.
Most often, fetuses known in utero as twin A are also first born and thus retain their label. However, this is not always the case, such that the twin A newborn was previously labeled twin B in utero and vice versa.
For example, a woman that has a sibling that is a fraternal twin is 2.5 times more likely to have twins than average! However, for a given pregnancy, only the mother's genetics matter. Fraternal twins happen when two eggs are simultaneously fertilized instead of just one.
While there are risks associated with all twin pregnancies, dizygotic (fraternal) twin pregnancies usually bring about the least amount of complications, carrying the lowest risk of all types of twins.
Monoamniotic-monochorionic Twins
This is the rarest type of twin, and it means a riskier pregnancy as the babies can get tangled in their own umbilical cords. If you have monoamniotic-monochorionic twins, your healthcare provider will monitor your pregnancy closely.
Around two in three sets of twins are fraternal. Two separate eggs (ova) are fertilised by two separate sperm, resulting in fraternal or 'dizygotic' (two-cell) twins. These babies will be no more alike than siblings born at separate times.
Fraternal twins may look more similar than non-twin siblings because they are the same age, and because they share the same environment (such as prenatal and postnatal nutrition).
However, for a given pregnancy, only the mother's genetics matter. Fraternal twins happen when two eggs are simultaneously fertilized instead of just one. A father's genes can't make a woman release two eggs. It sounds like fraternal twins do indeed run in your family!
Identical (i.e., monozygotic, or MZ) twins share 100 percent of their genes, whereas fraternal (i.e., dizygotic, or DZ) twins generally share only 50 percent of their genes.
Despite the name, identical twins are rarely completely identical. Their skin tone, weight, height or personality, to name a few characteristics, may be different. How does this happen? Every time a cell divides — as occurs in the developing embryo — your DNA must replicate, a very complicated process.
Factors that increase the chance of twins include: consuming high amounts of dairy foods, being over the age of 30, and conceiving while breastfeeding. Many fertility drugs including Clomid, Gonal-F, and Follistim also increase the odds of a twin pregnancy.
The rate keeps rising for (naturally) two big reasons. More twins are being born now than ever before. Parents are increasingly using in-vitro fertilization and deciding to have children later in life. The twin rate could continue to go up as more nations push these factors higher.
The notion that twins always skip a generation is also a myth. The illusion may have arisen because men who inherit the gene from their mothers are unaffected by it ( they do not ovulate), but can still pass it on to their daughters, who, in turn, will have an increased likelihood of conceiving twins.
There is no right or wrong answer, it's a personal preference. Some chose to be completely open or tell their twins to give a “smaller” twin leverage if he/she is older. Other choose not to tell so that they don't bicker about the subject or the dominate “older” twin doesn't have more leverage.
For vaginal delivery of both infants, the twins decide birth order by their placement in the uterus. Once the babies are large enough to stay in one position in the womb, the twin lowest in the uterus is known as Baby A and the one furthest from there is Baby B, according to the Stanford Medicine News Center.
The results suggest that twin fetuses are aware of their counterparts in the womb, that they prefer to interact with them, and that they respond to them in special ways. Contact between them appeared to be planned—not an accidental outcome of spatial proximity, says study co-author Cristina Becchio of Turin.
What is the likelihood of having more than one set of twins? If existing twins are identical the chance of having another set is the same as most women, about 1 in 250. If the twins are non- identical the chances of having twins again are much higher.