Some factors contributing to the increased rate of having multiples is becoming pregnant in the 30s or 40s and/or assisted pregnancy. Particularly the drug Clomid has been linked to an increase in the number of twins and triplets.
In order to be able to conceive non-identical twins naturally, there must be the gene in a woman’s body from her father or mother. The ability to conceive dizygotic twins is a female related issue. On the other hand, having identical twins has nothing to do with the genes!
Internationally, identical twins occur four per 1000. However, the statistics of having fraternal twins varies with race. Africans have the highest rate of dizygotic twins, with Europeans in the middle and the lowest rate of fraternal twins happening in Asia.
There are basically two types of conception that can happen with twins. (some believe there is a third type of twins, read more ..)
1) A single egg is fertilized by a single sperm and between 3 to 15 days after conception it splits and the result is identical or monozygotic twins. If the split happens by day 5, the twins will have two amniotic sacs, two chorions, even two placentas. If the split happens between day 5 and 9, then the twins will have one chorion, one placenta and two amniotic sacs. If the split happens 9 days after conception, then the twins will share one chorion, one placenta and amniotic sac. If division happens later on, then conjoined twins are formed.
Identical twins normally share one placenta, but each can have their own amniotic sac. However, a small percentage of identical twins have two placentas. In that case you may be under the impression that you will have dizygotic twins and get a surprise after birth.
The Placenta’s function is to deliver nutrients to the babies. There can be complications for babies who share a placenta. Of course identical twins are the same gender and share the same DNA. However they will have their own unique fingerprints!
2) Two eggs are fertilized by two sperms and the result is fraternal, non-identical or dizygotic twins. Fraternal twins usually do not share placentas. Each have their own placenta and amniotic sac. They are basically womb neighbours. Having said that, in some cases, the two placentas can fuse together and give the impression of being one placenta. In all reality, you can only be sure what type of twins you are having after you give birth.
Just for you to know Mary-Kate and Ashley Olsen are actually fraternal twins?!!
If you have not heard about the black and white twins you should watch this movie.
Note: Some fraternal twins look almost identical. Twin Zygosity Test is a popular way to find out if your babies are in fact identical. The test is very easy to use. You can prepay you order, receive your DNA testing kit, send back the DNA and receive the result within a short period of time.