Types of Twins - differences between identical twins and fraternal twins

Types of Twins- many people do not understand the differences between the different kinds of twins.

I can't believe how many times people will see my boy and girl twins and then ask me if they are identical. It's #1 on my list of stupid twin questions. No - they are not ! Check out the list for good laugh. Identical means they look alike - they match - and being that they are boy with straight brown hair and girl with blond curly, they obviously do not - seriously - they barely look related !(Can you tell I've been asked a million times ? )

People are fascinated with the idea of twins, but not necessarily educated about them. What they are really asking is about Zygosity So here is a brief, easy to understand page about the differences in the two main types of twins. You will want to know the differences both for yourself as parents, and for all those questions you will get from others. People will ask !

If you're pregnant right now, this is probably something that is on your mind and if you already have twins, then you know what I mean about the questions! Now there are some unusual cases -

Regarding Boy Girl Identical Twins

But for the usual twin types....

Here it is in a nutshell - There are two basic twin types - monozygotic - otherwise commonly known as identical - and dizygotic, commonly known as fraternal. (Yes there are also conjoined twins, but that is a whole separate issue.)

Monozygotic twins start out life the same way as a singleton ( one baby) - one egg and one sperm creating a zygote or fertilized egg. The zygote then splits an extra time creating 2 zygotes sharing the same genetic information. Approximately one-third of all twins are monozygotic . They are always the same sex and are usually very similar ( identical to the eye) in their basic physical features although they can have some variations in coloring or birthmarks etc.

About 25% of these twins are 'mirror' twins where they have similar but opposite features . One is left-handed, one is right -handed, their hair may part to different sides or they may have matching birthmarks on opposite sides of their bodies.When their baby teeth come in , they will come on different sides of their mouths. They will suck opposite thumbs. It's fascinating!

Some more 'terminology' regarding identical twins

If the split happens within 3 days of conception the identical twins will have 2 separate placentas, 2 chorions( inner membrane) and 2 amnions ( outer membrane).This is called a diamniotic, dichorionic twin pregnancy.

If the split occurs 4-7 days after conception the twins will have separate amnions but share a chorion and their placentas will be fused together. This is called a diamniotic, monochorionic twin pregnancy.

If the split happens after the 8th day the twins will share the same placenta, the same amnion and the same chorion. This is called a monoamniotic, monochorionic twin pregnancy.A concern with this type of pregnancy is that the twins may not get equal access to the nutrition from the placenta. They can be at risk for such problems as twin-twin tranfusion and some growth retardation.

About two-thirds of all types of twins are fraternal - 50% are boy/girl and 25% girl/girl , 25% boy/boy.

Dizygotic twins are thought to occur when the mother releases more than one egg in one ovulation cycle and the eggs are fertilized by separate sperm. Fraternal twins always have separate placentas,chorions and amnions although if they embed in the lining of the uterus too closely together the placentas may fuse together.

Dizygotic twins are more likely than monozygotic twins to have similar birth weights to each other. These twins are really no more alike than 2 babies from completely different pregnancies. They do not look any more alike than other siblings in the family would and would not have the same personalities or IQ's or rate of growth than other children.

Before your twins arrive you may be anxious to know their twin type. Some prenatal tests such as amniocentesis or chorionic villus sampling can determine the type by the second trimester. Talk over your options with your doctor or healthcare provider. DNA for accurates types of twins testing can also be done after birth.

Ultrasound usually provides the fastest answer although sometimes you still end up waiting until the birth of the babies to know for sure ! Blood tests done after the birth and careful examination of the placenta(s) can give you a definitive answer.

Why the increase in twins ??

Quite a few reasons. Some more are just more genetically prone to releasing more than one egg per cycle and this tends to run in families. The male partners family history has no bearing on this.( You will get asked this as well - " whose side of the family do twins run in ?") Only the mother's family history makes a difference here. It's your chance to explain and tell them about the types of twins too !

Another factor is that many women are waiting much later in life to have children. Age plays a big factor. Women 35 and older are far more likely to have fraternal twins than their younger counterparts. There is some research to suggest that the use of oral contraceptives can contribute as well as a women's chances of having twins go up dramatically if she gets pregnant the first month after stopping the pill.

The biggest cause today however, is the the increased use in fertility treatments. Because so many women are waiting longer to start a family, many are having problems with conceving. Technologies such as in-vitro fertilization essentially transfer a greater number of eggs in hopes that at least one will develop. Many times it is more than that.

Study up a little on the types of twins - you will be an expert in answering questions by the time the babies arrive. People are fascinated by twins.

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