This is a very good article on twins. Wikipedia has some stats on twins as well and describes very well on the history behind.
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Twin Pregnancy
How You Get Pregnant With Twins
By Robin Elise Weiss, LCCE
"Types of Twins"
People are very fascinated with twins and other multiples. It seems mysterious how one pregnancy can result in more than one baby.
Multiple pregnancies are on the rise in recent years with more and more twins and other types of multiples being born. The majority of the reason for this rise is the increased use of fertility drugs.How Twins Are Made
There are a couple of ways that multiples are conceived:
Multiple eggs are released or there is more than one ovulation. Both (or more) eggs are fertilized and you have fraternal twins. This can happen with or without fertility drugs.
One egg is release but splits in two. Identical twins are made. This can happen with or without fertility drugs.
In cases of IVF, there are usually 3 or more fertilized eggs put back into the uterus. The rationale is that not all of these will implant, though sometimes they do.
Once conceived you have a lot of possibilities on placentas, implantation locations, number of amniotic sacs. For example, you can have two babies, two placentas, and two sacs. You can have two babies one placenta, two sacs. It will be important to know exactly what you are dealing with, particularly when it comes to amniotic sacs.
How Multiples Are Discovered
Many people learn about their multiples at an early ultrasound. Although some critics say that before twelve weeks gestation there is still a high risk of Vanishing Twin Syndrome (VTS), where one baby stops growing and is either absorbed back into the mother's body or is still born when the other twin is born.
Some people find a rapid rate of grow in their uterus which leads some to suspect twins. I will say, for all of you second timers out there, that your bellies will grow faster than with your first pregnancy, so don't automatically assume twins.
Sometimes multiple heart beats are heard, leading your practitioner to believe that there is more than one bundle of joy in your uterus.
Some women find out when they have abnormally high levels on the AFP test that multiples are on the way, since multiple babies would increase the levels of these hormones. It's also sometimes possible to detect multiples with serial hCG levels in early pregnancy. The more babies the faster the rate of rise in the hCG which should nearly double every 48 hours with a singleton.
About 3% of all twin pregnancies are undetected until birth. It is possible to have an ultrasound and not see the other baby, though rare.
http://pregnancy.about.com/od/pregnancy/a/Twin-Pregnancy.htmhttp://en.wikipedia.org/wiki/Twin-----------------------------------------------------------------------------------------------
Twins are apparently on the rise in a major way.