One famous question asked by pregnant women carrying twins is if the twins are identical or fraternal. Ultrasound can only show the gender of the babies. If the gender is different then it is a fraternal twin but if the gender is similar it can either be fraternal or identical. Ultrasound might suggest that similar gender means identical twins.
A placenta is shared between two different amniotic sacs if the egg divided between the 3rd and 8th day after fertilization. This is known as diamniotic. It can also be seen as 2 embryos are formed with individual placentas and amniotic sacs when an egg splits into 2 after fertilization. When this happen, a fraternal twin can be seen through ultrasound but if the gender for two babies are the same, it could be fraternal or identical twins.
The chances of you carrying identical twins is when they share one placenta but remember that looking at the number of placenta is very difficult. Another way of seeing the types of twins also depends on the thickness of the membrane which separates the sacs. The membrane is thin if it is one placenta and if it is a separate sac then, the membrane is thicker.
Monoamniotic twins are those which share a single placenta and amniotic sacs. An identical twin can be seen through ultrasound by seeing this feature. The chances of this happening are rare as it only happen in 1 in every 60,000 pregnancies. Siamese or conjoined twin is formed when the egg is divided after thirteen days of being in the womb.
Although it is difficult to be sure about the types of twins a woman is carrying, a sonographer uses a precise signs in this process. The most common way to see if the twins are fraternal or identical is by looking at the placentas. This technique is easiest in the first trimester.
Figuring out the types of twins is important due to the fact that different twins have different risk and problems. Only by knowing the types of twins a woman is carrying enable tests to figure out what the problem is. Test done on amniocentesis, chorionic villus sampling (CVS) or fetal blood sampling are known as zygosity studies.
I hope this article about fraternal or identical twins are useful to you. But on a personal level, I don't mind if ever I have twins - whether its fraternal or identical. The important thing is both are born healthy. How about you?