These drugs work by promoting ovulation. It therefore follows that twins or higher-order multiples will be non-identical, as they result from fertilization of two (or more) separate eggs.
When multiple pregnancy follows the use of clomiphene (clomid), as happens with about 10 per cent of users, it is mostly twins. When it occurs as a result of IVF treatment, rates are higher at 25 to 30 per cent; the chances of higher order multiples such as triplets or quads are also significantly increased.
The occurrence of identical twins among fertility treatment users is the same as in the general population, i.e. about one in 250 births.
Can doctors tell whether twins are identical before they are born?
Only if they share the placenta or the sac. To establish with an acceptable degree of certainty whether or not twins share a placenta, an ultrasound scan needs to be performed rather early, probably before sixteen weeks of gestation.
Later on, two placentas, which are close together, may appear to be virtually continuous and one cannot say with absolute confidence whether they are separate or not.
A single sac with two fetuses is a sure certainty that they are identical and can be confirmed at any stage of the pregnancy by ultrasound scanning. Only about 1 per cent of identical twins share the same sac.
Non-identical twins never share a placenta or gestational sac.
Not at all. All non-identical twins have separate placenta but so do 30 per cent of identical twins. If the division of the egg occurs early, say within three days of fertilization, this division is total and the two fetuses will have separate placentas and sacs. This is why establishing whether twins are identical or not is not a straightforward affair before they are born. Of course, if the twins are of different sexes, this confirms that they are non-identical.
The proper term is "conjoined twins". The mechanism is still the same and it is all to do with the timing of division. If this occurs very late, at about the twelfth day or later after fertilization, the division will be incomplete and the twins will be joined and will share some parts of their bodies.
Conjoined twins are therefore identical twins, where division occurred rather late and was therefore incomplete.
In virtually all cases, yes. It is, however, possible to miss the diagnosis if they only share a thin pliable piece of skin, as this may not be immediately apparent on the ultrasound scan.