Different Types of Twins

When it comes to pregnancies that involve twins, most individuals are familiar with fraternal (dizygotic) or non-identical twins and identical twins (monozygotic) as these are the most commonly occurring types of twins. Fraternal twins occur when there are two fertilized eggs that get implanted into the uterine wall and both of them become fertilized by two different sperm. Identical twins, on the other hand, occur when a single egg becomes fertilized by one sperm but the zygote divides into two separate embryos. With fraternal twins, there is a very small chance of the two babies having the exact same chromosome profile, however, with identical twins they may share some percentage of chromosome profile depending on when the embryo split. With this said, let’s take a look at a few types of twins that you probably didn’t know about!

1. Conjoined Twins

These types of twins are classified under the monozygotic category and are considered multiples of one another due to an incomplete division of the fertilized ovum. Generally, these types of twins are connected at some point of the body such as the torso, head, or arms and may share organs and tissue.

2. Twins Conceived Separately:

When a woman has an egg fertilized by sperm, her cycle will become interrupted and ovulation will cease, which is how she will know that she is pregnant. On rare occasions though, the cycle will release another egg while the woman is already pregnant. Theoretically, if this egg becomes fertilized from the same man’s sperm, then this can result in twins that are conceived at different times. In medical terms, this is called superfetation.

3. Twins With Different Fathers

Fraternal twins with different fathers are the result of hyperovulation, where the woman has multiple eggs released in a single cycle. These eggs are fertilized by two separate incidences of sexual intercourse and when fertilized by sperm from two different partners, this results in fraternal twins with different fathers.  In the medical world, this is known as heteropaternal superfecundation.

4. Half-Identical Twins

These type of twin is more of a theory than an actually confirmed diagnosis. The proposed theory goes that half-identical twins could be very much alike but not necessarily match 100% on DNA. How would this work? Two eggs would have to be fertilized (fraternal twins) with both of those eggs splitting (like with identical twins) and each half of either split egg then meets the sperm = polar body twins.

5. Mirror-Image Twins

These types of twins form from one single fertilized egg. When this type of split occurs more than a week after conception, the twins will almost always develop reverse asymmetric features. Although these aren’t considered a real type of twin, they are a sub-category that describes a single event that can happen to monozygotic twins.

Other types of twins include parasitic twins which are a type of conjoined twin, semi-identical twins which is when the twins are identical on the mother’s side but only share half of the father’s genes and twins that come from different races, which are a type of fraternal twin that have different racial characteristics. In most cases, in order to know whether your twins are identical or fraternal at birth, a DNA test must be completed.