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Twin types - differences between identical twins and fraternal twins
Twin types- many people do not understand the differences between the types 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 and girl, they obviously do not - um - yeah - one has a penis ?(Can you tell I've been asked a million times ? ) 
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. 
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 somegrowth retardation. 
About two-thirds of all twin types 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 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 examinationof 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 twin types 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 arewaiting 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 twin types - you will be an expert in answering questions by the time the babies arrive. People are fascinated by twins.
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