essay written by Mark Sanders, Twinstuff.com

Believe it or not, that's one of the most popular questions we receive at twinstuff.com. While there is no single way to determine whether or not you're going to have multiples (until after the ultrasound and/or delivery), there are ways to at least give you a decent guesstimate.

Use these charts for a rough estimate on the chances of having twins or triplets. While we can't vouch for the complete accuracy of these percentages, they are based on the numbers available on twin births from the National Vital Statistics Reports (1997) and (1999). Some of the numbers below are also courtesy of the Facts About Multiples Web Site.

Note that the numbers for specific age groups are for all reported twin births in the United States - they take into account both twins conceived naturally and those via fertility drugs or IVF treatment. As a result, the odds of conceiving twins naturally - and carrying to full term - are most likely lower than represented below.

And remember, just because mom or pop is a twin doesn't mean two are on the way. There is no way whatsoever to predict identical twins so a mom who is an identical twin has no better chance of producing twins than any other women. And the twinning gene is carried on the female side, so dad doesn't have much to do with it (at least in terms of the first generation - the dad can pass along a gene to his daughter).

The general rules of thumb are the older the mom, the better chance of having twins. And if fraternal twins are on the women's side of the family, your odds increase dramatically as well (and as you can see below, don't think that delivering one set of fraternals means you're immune from delivering a second set).

General Odds of Having Twins

Twins of any type - 1:40
Spontaneously fraternal twins - 1:60
Identical twins - 1:240
2nd set of identical twins - 1:62,500
Two sets of fraternal twins - 1:5
If mom is fraternal twin - 1:20-25
Conjoined twins - 1:60,000
Twins in Australia - 1:66
Twins in UK - 1:72
Twins in Canada - 1:89
Twins in Japan - 1:166
Twins in Nigeria - 1:22
Twins if using Clomid - 1:5

Ethnicity
White under 20 - 1:66
White between 20 & 24 - 1:50 (1997)
White between 25 & 29 - 1:39 (1997)
White between 30 & 34 - 1:31 (1997)
White between 35 & 39 - 1:27 (1997)
White between 40 & 44 - 1:23
White between 45 & 49 - 1:6
White between 50 & 54 - 1:3
Black under 20 - 1:54
Black between 20 & 29 - 1:33
Black between 30 & 39 - 1:28
Black between 40 & 49 - 1:38
Hispanic under 20 - 1:80
Hispanic between 20 & 29 - 1:56
Hispanic between 30 & 39 - 1:39
Hispanic between 40 & 49 - 1:42

Triplets
Having triplets in U.S. - 1:517
Spontaneously conceived triplets - 1:8,100
White under 20 - 1:6,800
White between 20 & 29 - 1:1,100
White between 30 & 39 - 1:350
White between 40 & 49 - 1:290
Black under 20 - 1:3,600
Black between 20 & 29 - 1:1,600
Black between 30 & 39 - 1:740
Black between 40 & 49 - 1:570
Hispanic under 20 - 1:5,100
Hispanic between 20 & 29 - 1:2,200
Hispanic between 30 & 39 - 1:810
Hispanic between 40 & 49 - 1:700