Embryo companions When you were in the process of implanting in the side of your mother's womb, you may not have been alone. Your mother may have ovulated more than once and there may have been one or more other blastocysts developing alongside. They may have been there a day or so before you or arrived a day or so afterwards
Identical twins
Now it can happen that another tiny human was developing alongside you that was made from the same fertilised egg as yourself: an identical twin. As soon as the DNA from the sperm and egg unite on that fateful first day, they begin to divide. Each cell divides into 2, this 2 into 4, those four into 16, that 16 into 256, that 256 into 65536 and so on, until a tiny bundle of cell is formed. Now sometimes whatever it is that keeps these cells gathered into one bundle fails, and the bundle splits into two. Then each little bundle develops normally into a baby. The result is two genetically identical little babies. As things are happening so fast, the day in which the bundle of cells splits into two is critical to how things work out.
A chorion, amniotic sac and a placenta for each twin
If the split occurs in the first 3 days following fertilisation, i.e.. before the yolk sac is formed, then the twins will each have their own placenta, amniotic sac and chorion. This is uncommon, but it is possible. If two separate placentas are seen on ultrasound this can mean identical twins.
A shared placenta but a chorion and amniotic sac for each twin
If the split occurs between the 4th and 7th day, ie after the yolk sac is formed, the twins will have their own chorion and amniotic sac, but will share a single placenta. This is the most common eventuality.
A shared placenta and chorion, but an amniotic sac for each twin.
If the split occurs between the 7th and 9th day, i.e., some day after the yolk sac is formed, the twins will share the placenta and the chorion, but each have their own amniotic sac.
A shared placenta and both twins in one chorion and amniotic sac
If the split occurs after the 9th day, ie. a day or two after the yolk sac is formed, the twins will be grow together in a single sac with a single, shared placenta. This is quite rare, seen in only 0.1% of identical twins. these are known as “monoamniotic” twins, which means that both babies share the same living space.
Identical twins always have the same DNA and are the same sex. They have similar hand and foot prints, but they do differ, with different fingerprints and teeth marks. The birth rate of monozygotic twins is constant throughout the world (approximately 4 per 1000 births.)
"Mirror" twins
Once in every four identical twin births, the babies are literally reflections of each other. This is called "mirror imaging" and results from a delayed splitting of the egg (day 7 or beyond.) Everything is opposite: one is right-handed and the other is left-handed. Occasionally, one “mirror” twin's heart is on the right side and the other as normal, on the left. Mirror twins are formed when the original right half of the egg becomes one individual and the original left half becomes the other. These twins will often have mirror images of their features, such as hair whorls that run clockwise in one and counter clockwise in the other, a birthmark on the right shoulder of one and the left shoulder of the other, etc. It is interesting to note that a little over one third of identical twins are left-handed, double the rate in the general population.
More than two identicals
Some times the little bundle of cells splits twice, making four little bundles of cells. If this happens, which is very rare, then usually one of the embryos dies away and three are left: identical triplets. Another way in which identical triplets can be formed is when only one of the bundles divides a second time, which creates three genetically identical babies.
Even more rarely, all four little bundles develop into four identical babies. For these to survive to birth is very, very rare indeed. It has been suggested that among all the countries of the world there are less than 30 sets of identical quads alive today. As is the case with identical twins, these developing embryos also may share the placenta or amniotic sac or not, depending on at what stage the bundle of cells splits.
Fraternal twins
When two eggs are fertilised in the same month, the embryos develop together side by side in the womb, each in its own chorion. They are called “fraternal“ twins. There may be more than two embryos were present, especially if several embryos were introduced to the womb as part of IVF treatment.
Assisted reproduction
There is an epidemic of fraternal twin births in the Western world at present because of assisted reproduction techniques. The rate of twinning has increased markedly since 1978 when the first “test tube” babies were conceived outside of the womb. For more than twenty years it has been possible for women to take drugs to stimulate ovulation. Several eggs are produced in a month, so there is a greater chance that more than one will be fertilised.
Women who have had difficulty conceiving have IVF, where several eggs retrieved from the womb are placed in a dish and mixed with fresh sperm. Several of the healthiest-looking ones are placed back into the womb, but the incidence of multiple births is now so high that this is causing concern in medical circles. Multiple births are risky to both mother and baby.
The incidence of twinning
Fraternal
The number of fraternal twins born varies greatly throughout the world. The highest twinning rate in the world is in the Yoruba tribe of Nigeria. It is slightly lower in the UK than in the USA. Asian countries have the lowest twinning rate. The twinning rate for fraternal twins in Japan is only 1 in 166.
Identical
Identical twin births are constant throughout the world at 1 in 250 live births. No one is sure why the fertilised egg splits into twins, but this does not seem to have anything to do with geography, diet or anything else. It is not genetic but there is a slight chance that the tendency is inherited. Recently it has come to light that the rate of identical twinning is increased among families who conceive through IVF, possibly because of zonal manipulation, which is carried out on the egg to assist penetration by the sperm. As IVF pregnancies all begin with more than one embryo present in the womb, it is my belief that it could be the actual presence of other embryos that causes the splitting of the egg into two.
These statistics relate to born twins, not conceived twins. The number of embryos conceived does not reflect in any way the number that come to birth, or even the number that can be seen on ultrasound. This is because, before even the first ultrasound scans are made, many of them have gone. Yet we know we remember them in the Dream of the Womb.
Althea
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