# What is the probability of having a baby boy and girl together?

Asked by Izzard (70) December 20th, 2007

My wife is pregnant with twins, one of each gender. I’d like to know what the probability is of this. I.e. in how many pregnancies is one pregnancy a girl/boy pair on average? Just to stir things up a bit: I am white/English and my wife is black/Zimbabwean.

Observing members: 0 Composing members: 0

This question intrigued me, so I checked out the census data online (I am really procrastinating at work!). It looks like 3.1% of births are twin births in the U.S. (the most recent data is 2002…i suspect this may have gone up in the past 5 years due to the prevalance of hormonal treatments for infertility, which often lead to multiple births). You can access the data here. Of course, this is only for US births, so that won’t help if your question relates to the frequency of such births in the UK or Zimbabwe. Also, this data doesn’t differentiate between identital twins (same sex) or fraternal twins (which can be same sex or different sex).

sfgal (283)

oops, here is a link the stats: http://www.cdc.gov/nchs/births.htm

sfgal (283)

This is not an answer but a congratulations (and an ouch). So exciting; obviously you know the genders but what fun to see their appearance. Do keep us posted. When is her due date?

My grandmother had fraternal boys who did not resemble each other; one of my step- sons has fraternal one-of-each, altho they both have very blond curls and blue eyes.

gailcalled (54644)

Assuming we’re ignoring potentially affecting factors (such as whether the mother was taking fertility treatments or was over 35), the statistic I found online was that approximately 1 in 60 births is a fraternal twin birth. There are four outcomes (boy-boy, boy-girl, girl-boy, and girl-girl), of which two have mixed genders, so it follows that half of fraternal twins should be mixed genders. So I would expect around 1 in 120 births would have this outcome.

I have absolutely no idea if race has anything to do with it. I believe that if there are other fraternal twins in the mother’s family she has a higher chance of having them. The mixed genders seems like it should be completely arbitrary, but I don’t know any hard statistic on it.

finkelitis (1907)

Thank you folks. The babies are due in mid-March but likely to be earlier than that (being twins).

Finkelitis, your logic agrees with what I was thinking. I’ve seen lots of figures for fraternal twins – from 1 in 40 to in 1 in 90. These days they’re more likely because of fertility treatments, as you’ve suggested. And I’ve confirmed that about 50% of fraternal twins are same-sex and 50% are boy/girl. So I think I’m satisfied with a figure between 120 and 180. (I just wanted to know how special they were. :) ) Thanks for your help!

Izzard (70)

You bet. Congratulation!

finkelitis (1907)

In families without a history of twins, age and overall health can also cause twinning, although I don’t know if identicals or fraternals are more likely or not. My grandmother delivered identical twin daughters in her last delivery. I believe there was between 10–15 years between her first and last and she had about 7 or 8 kids. None of her offspring produced twins. Sorry, my genealogical stats are buried somewhere and it IS past my bedtime:)

gcross (272)

Congratulations on the twins! I’m in the same boat, though we don’t know the gender as of yet. Suggestion: If you’re not already doing so, keep a journal of your thoughts and what’s going on as you await the big day. It’ll be great to share this with the kids when they’re older and wonderful for you to reflect upon in years to come.

Blessings to you and your family!

or