Chances of having twins and what affects them

[Nefertitties]

FACTORS AFFECTING TWINNING
These factors describe the mother.
Race: Black women have a higher rate of twinning than white women. Asian women have the lowest rate.
Age: Twinning rates are highest between ages 35-39, after 40 the rate decreases except for black women, where the rate increases until age 49.
Sexual activity: Women with higher sexual activity have a higher rate of twins.
Height/weight: Taller and heavier women have a higher rate of twins.
Previous children: Women with more children are more likely to have twins. A 4th or 5th pregnancy is four times more likely to be multiples.
The Pill: Shortly after going off birth control, hormonal fluctuations cause many women to ovulate twice.
Heredity: Fraternal twins, actually the inclination to ovulate more than once per cycle, seems to run in families, although no one has discovered a gene (see below in Chances of Conceiving Multiples for more information). Twinning does not skip a generation, and just because someone has fraternal twins in your family doesn't mean you will.
Fertility treatments: Hormonal treatments with drugs and surgical reproductive treatments have dramatically affected the twinning rate.

There is nothing you can do to increase your chances of having twins, and there is no clear way to predict twins, regardless of family history, fertility treatments, etc.


CHANCES OF CONCEIVING MULTIPLES
Chances of identical twins: 1 in 285 (figure is universal, identicals happen randomly)
Chances of fraternal twins: 1 in 43 (higher or lower in some countries - in Japan, it is 1 in 149 while in Nigeria it is 1 in 25)
Chances of conjoined twins: 1 in 50,000 (please note this is the conception rate, not the birth rate)

Chances of more than one set:
2 sets of fraternal twins - 1 in 3000
1 set of identical/1 set of fraternal - 1 in 10,000
2 sets of identical twins - 1 in 70,000

Chances if YOU are a twin:
If you are fraternal: 1 in 17
If you are identical: this does not affect chances, so 1 in 38-90

If your mother is a twin, you may have a higher than average chance of twins. If your father is a twin and you are female, it may slightly increase your chances. Being married to a twin does NOT affect your chances. If your parent is an identical twin, your chances of having twins are the same as anyone else's.

Chances of spontaneous triplets, in the US: 1 in 8100 for white women, 1 in 9800 for black women

US triplets and higher rate (all higher-order multiples): 1 in 539

Fertility treatments have drastically affected the rate of higher-order (4+) multiples being conceived. From 1900-1950, only 46 sets of quads were born worldwide. In 1994, at least 79 sets of quads were born just in the US.

Average rates of higher-order multiples:
102 sets of quads are born every year in the US
12 sets of quints are born every year in the US
1 set of sextuplets are born yearly in the US

1 in 5 Clomid pregnancies are multiples. Clomid, an ovulation drug, will increase your chances of conceiving multiples by roughly 10%.