Types of Twins

Two girl twins can be either identical or fraternal (monozygotic or dizygotic). Two boy twins can be either identical or fraternal (monozygotic or dizygotic). A set of boy/girl twins can only be fraternal (dizygotic).

Conjoined twins develop when an early embryo only partially separates to form two individuals. Although two fetuses will develop from this embryo, they will remain physically connected — most often at the chest, abdomen or pelvis.

Because fraternal, or dizygotic, twins are 2 separate fertilized eggs, they usually develop 2 separate amniotic sacs, placentas, and supporting structures. Identical, or monozygotic, twins may or may not share the same amniotic sac, depending on how early the single fertilized egg divides into 2.

Twins and parents of twins get this question all the time, and it's an easy one to answer. Except in one situation. In 2007, scientists discovered a pair of "semi-identical" twins. That's right – this brother and sister are half identical and half fraternal, says Melissa Parisi, a pediatric researcher with the National Institute of Health.

Fraternal twins are “dizygotic,” meaning that they developed from two different eggs fertilized by two different sperm cells, while identical twins are “monozygotic” i.e., they developed from a single fertilized egg that split.

Mirror twins—or mirror image twins—aren’t really a category of twins, like identical/monozygotic twins. Rather, the term “mirror twins” describes a characteristic of some twins, where their features appear asymmetrically -- that is, on opposite sides.

There are 7 different kinds of twins, categorized as: Identical, Fraternal, Half-Identical, Mirror Image Twins, Mixed Chromosome Twins, Superfecundation and Superfetation. pixabay Normally, except the first two kinds, others are known to be quite rare to occur.

Parasitic or Heteropagus Twins are asymmetric conjoined identical or monozygotic twins in which the tissues of a severely defective twin (the parasite) are dependent on the cardiovascular system of the other, largely intact twin (autosite) for survival.

Explore instances of polar body twins, parasitic twins, and mirror images twins. Twin births are common, though there are rare cases of unique twins. Explore instances of polar body twins, parasitic twins, and mirror images twins.

Superfetation differs from the conception of twins, where multiple ova are released in a single cycle. This can happen naturally or be stimulated with fertility drugs. When more than one ovum is fertilized and implants in the uterus, the result is dizygotic twins, triplets, or other multiples.

Monozygotic twins cannot have different fathers. However, fraternal twins are the result of hyperovulation, the release of multiple eggs in a single cycle. Superfecundation describes a situation where the eggs are fertilized by sperm from separate incidences of sexual intercourse. In a case where a woman has sex with different partners, the twins could have different fathers.