When a woman becomes pregnant, the process of conception does not always stop at one baby. A pregnancy of more than one fetus concurrently is called a multiple pregnancy. While the majority of multiple pregnancies refer to twin/triplets, with the increase in the use of fertility drugs, multiples have been growing in number in recent years.
There are various situations that may lead to the conception or growth of a multiple pregnancy. For the sake of discussion, the twin/triplet pregnancy is easiest to understand.
When a female releases more than one egg, when she ovulates twice or she ovulates three times two/three eggs and all eggs are fertilized, dizygotic twins/trizygotic triplets (fraternal or nonidentical) twins/triplets are created.
Dizygotic or Fraternal twins/triplets are the most common form of twin/triplet birth. They happen when separate eggs are fertilized by separate sperms. The incidence of dizygotic twins depends on the woman's ethnic backgroud. Africans have a higher incidence, Caucasians are somewhere in the middle, and Asians have the lowest incidence. Contrary to common belief, fraternal twins/triplets do not have to be of opposite sexes. Fraternal twins/triplets can have differing blood types, hair color, eye color and even body type upon birth. All dizygotic fraternal twins/triplets will have different amniotic sacs and, most often, different placentas.
On the other hand if one egg is released with only one sperm fertilizing the egg and the egg then splits into multiple embryos monozygotic or maternal/identical twins/triplets are created. This splitting of the fertilized egg, the zygote or blastocyst, takes place usually during the first week after fertilization or sometimes even later. The result depends on exactly when the splitting happens:
The stage at which the egg cell splits determines how the twins will implant in the uterine lining, and whether or not they share an amnion, chorion, and placenta. Basically, the earlier the splitting occurs, the more independently the twins will develop in the uterus. So, a pair of identical twins that split during the two-cell stage will each develop its own amnion, chorion, and placenta.
Twins that split during the late blastocyst stage will share an amnion, chorion, and placenta.
Zygosity is the degree of identity in the genome of twins.
There are five common variations of twinning. The three most common variations are all dizygotic (fraternal):
Monozygotic or identica/maternall twins/triplets are far less likely to occur based on the fact that the single egg splits into two or three embryos during the first stage of development. The incidence of spontaneous monozygotic twins is the same all over the world about 1 in 250 pregnancies, though it increases if the pregnancy was achieved with IVF in-vitro fertilization. Identical twin/triplets may share a placenta or an amniotic sac or they may have separate placentas and sacs. All identicals will have the same blood type, hair color, eye color and sex. Contrary to common belief, identicals do not have the same fingerprints or always the same DNA. Monozygotic twins can have two different outer and inner sacs (Dichorionic/Diamniotic), one outer and two inner sacs (Monochorionic/Di amniotic), or they can both be in the same sac (Mono/Mono). Twins that share the same outer sac are at increased risjk for certain complications because they can share blood vessels.