What are my odds of having a boy?

You may have heard it from your know-it-all in-law, or a snidely friend who happens to have one-of-each, or maybe even your doctor: after having 2 or 3 boys, you might as well forget about ever having a daughter, because the odds are you're going to keep having boys. Or if you've only had girls, maybe you "just can't make" a boy.

Well, think again. Remember that looking at very small data samples (two or three children) is very misleading: so let's take a look at data for over 6,000 American families, and find out:

• Do parents tend to keep having children to try for a boy or a girl?
• Are you more likely to keep having the same gender if you already have boys or girls?
• What are the odds of having all boys or all girls?

The 51/49 Boy/Girl Ratio

The ratio of 51% boys to 49% girls seen here is representative of overall US birth rates. There are many hypotheses about why there are slightly more boys born each year than girls, but no one knows for sure why this is so. Here are a couple of the theories:

• Males are more fragile (male infants are less likely to survive their first year, and a man's expected lifespan is less than a woman's). The slightly higher conception rate of males is nature's way of evening out the balance.
• As Dr. Shettles theorized, Y-bearing male-producing sperm may have a speed advantage over X-bearing female-producing sperm, more often winning the race to fertilize the egg and resulting in more male conceptions.

Regardless of the reason, the 51/49 ratio remains constant year to year throughout the US population.

Are Boys or Girls Preferred?

Do parents tend to keep having children in hopes of having a boy or a girl? In some cultures, there is a strong desire for a son, but parents in the US may have a "balance preference", a desire to have one of each gender. Here's what the data shows:

Did the first child's gender influence whether to have a second child?

• Having a second child doesn't seem to be influenced by the gender of the first child: the outcome is very nearly even, with parents being just slightly more likely to stop at one child if the child is a girl.

Did previous children's gender influence whether to have a third child?

• Parents are somewhat more likely to have a third child if the first two children are the same gender; slightly more so if both are boys.

Combining boy/boy and girl/girl, we see:

What about the fourth child?

• Surprisingly, parents 2 boys and 1 girl are the most likely to have a 4th child, even more so than parents of all boys or all girls.
• Conversely, parents of 2 girls and 1 boy are least likely to have a 4th child.

Combining this into same-gender and mixed-gender families, it turns out they are almost equally likely to have a 4th child, although same-gender families are slightly more likely.

The Odds of Having a Another Boy or Girl

Okay, finally! Let's look at the odds of having a boy or a girl, given that previous children are all of the opposite gender.

Odds of Having a Girl After 1, 2, or 3 Boys

The odds of having a girl seem decrease after having each boy, but only very slightly. Even after 3 boys, you are only 6.4% more likely to have a 4th boy than a girl.

Odds of Having a Boy After 1, 2, or 3 Girls

The odds of having a boy seem to increase after having girls, except after 2 girls, when a 3rd girl is more likely.

Gender and Birth Order

Given the data above, it looks like you are slightly more likely to have a boy, regardless of previous children. This is probably due to the overall 51/49 boy/girl birth ratio. This ratio, interestingly, varies slightly with birth order; it isn't consistent among first-borns, second-borns, etc.

What's it all mean?

Although we often hear the "statistic" that you are 30% or even 70% more likely to keep having the same gender, this is just an old wives tale. It is NOT a fact. The truth is, your odds stay pretty close to 50% for each child and only vary slightly. If you have had 2 or 3 boys, you are only very slightly more likely to have another boy. If you have had girls, you are slightly more likely to have a boy next. Don't believe that you can "only make" boys or girls -- most likely, it's only chance!

When am I most likely to get pregnant with a boy?

How likely is it to have a boy after having a girl?