[PNEWS-L] Twins from Two Different Fathers

[EpSil0n-//]

Secola Edwards
BellaOnline's Twins Editor

Monozygotic and Dizygotic Twins

Identical or Fraternal, or should we say monozygotic or dizygotic?

What does the term monozygotic twins mean?
What is the more common name for a monozygotic twins?
What are dizygotic twins?
What is the more common name for dizogotic twins?

Once it is revealed that a mother is expecting twins, one of the first 
questions that occur to her is whether or not the babies will be identical 
or fraternal once born. Monozygotic and dizygotic are the medical terms 
for identical or fraternal gestations.

Here are definitions and explanations of the two.

Monozygotic twins,, or more commonly referred to as identical twins, begin 
life as one egg, which becomes fertilized by one sperm and splits in to 
two eggs early in the gestational period. Identical twins are very rare, 
have no hereditary link and are almost always of the same gender (boy/boy 
or girl/girl). Less than one-third of live twin births are identical.
A little known fact is the physical similarities of identical twins depend 
on how soon the embryo splits after fertilization. The embryo splits very 
early on during pregnancy, most often within the first 12 days. An embryo 
that splits before day 4 will result in identical twins who appear much in 
the same way as fraternal twins, there could be very little physical 
similarity betw. In this case, a zygosity test may be needed to confirm 
the babies' identical status. A split between days four to eight will 
result in the babies sharing the same sac, making it easier to determine 
zygosity. Once the embryo reaches day 8 to 12 and splits during this 
timeframe, then the 'seeing double' effect takes place, resulting in twins 
who are very similar in appearance. Embryos splitting on or after the 12th 
day will most likely result in conjoined twins.

Dizygotic twins, or fraternal twins, begin life as two eggs fertilized by 
two separate sperm. The eggs are usually fertilized at 2 separate times 
during ovulation, but the time interval is very close. Dizygotic twins are 
the most common type of twin birth, with over two-thirds of live twin 
births being fraternal, especially with the increased use of fertility 
drugs, which almost always result in a fraternal twin pregnancy. Fraternal 
twins may be of the same sex or mixed-gendered, have strikingly similar 
appearance or look nothing alike, as in the case of my fraternal twins. 
And, yes, it is possible that fraternal twins can be fathered by sperm of 
two different men.

By the time an expectant mother has had her first ultrasound, her doctor 
should be able to discern whether the twins are in separate sacs or share 
a sac.



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