Twin Types

Twins are the development of two embryos in the womb. There are a few types of twinning but the two most common ones are fraternal and identical. Fraternal twinning which by far occurs more commonly is technically known as dizygotic. Monozygotic or identical twins are a rarer occurrence.

Dizygotic or Monozygotic? Microscopic study of the placenta or membrane alone is not enough to tell if your twin is dizygotic or monozygotic. Perhaps you can have an indication but only DNA testing can provide you with a conclusive answer.

Dizygotic Twinning in Detail

• Dizygotic (DZ) twins involve the union of two eggs with two sperms, resulting in two embryos. Two separate babies forms, who are genetically dissimilar as any two siblings born separately to the same parents.
• Conception of DZ twins occur when the mother produces two separate eggs in the same monthly cycle or when more than one embryo is transferred to the uterus during IVF.
• DZ twins usually have separate placentas which may at times fuse together resembling a shared placenta
• DZ twins can result because of superfecundation which is when conception takes place on two different occasions during the fertility phase. This would mean one fetus will be a few days older and therefore more developed.
• Fraternal twins run in families through the mother's side. Some women genetically ovulate more than one egg per cycle.
• Women who have had more than 3 children are at an increased tendency to conceive fraternal twins.
• Fraternal twins are more common in mothers aged 34-39 as ovulation is more erratic during these years.
• Fifty percent of all fraternal twins are boy/girl pairs, with twenty five percent being boy/boy and the other quarter being girl/girl.

Monozygotic Twinning in Detail

• Monozygotic (MZ) twins start off in the same way as any singleton pregnancy i.e. one egg unites with one sperm to form one embryo. BUT in the very early stages of cell division the developing cell group divides into two and progress as two separate embryos. It is not established why the split occurs.
• Genetically, MZ twins are coded with identical DNA. They look very similar but are two different people.
• Some MZ twins have two placentas while some share one.
• MZ twins have same genes but different fingerprints.
• The chance of having a pair of girls is the same as having a pair of boys.
• Identical twins do not run in the family, and so it can happen to anyone.

The Sub-types

Mirror-image Twins Mirror image twins have physical features are on the opposite side of their bodies. One twin being left handed and the other being right handed is a clear example. Other obvious examples include having birth marks or moles on the opposite sides. About 25% of MZ twins are mirror-image.

Conjoined Twins Popularly known as Siamese twins, conjoined twins occur in 1 in 100,000 births or 1 in 200 MZ twins. Conjoined twins can be identified via ultrasound in the early part of pregnancy. Apart from sharing placentas and membranes, conjoined twins also can also share body parts and organs. Occurrence of conjoined twins happens when the zygote divides late and doesn't split totally. The twins can be linked at any part of the body but both will be joined at the same place.