For centuries, predicting whether it’s a boy or a girl has been the sport of many grandmothers-to-be. With so many schemes, do any of them actually work?

My favorite is a scoring system based on more than 20 questions of the mom-to-be. These include:

  • Does your abdomen look like a watermelon or a basketball?
  • Are you having headaches?
  • What kind of cravings are you having?
  • If you hang a threaded needle over your belly, does the needle move in circles or side to side?
  • Do you pick up a mug by the handle or the body of the mug?

I’m not joking. Apparently how you drink your coffee or tea is a sign of whether you should paint the nursery pink or blue ...

Where do these ideas come from?

A lot of the ideas about how you can predict your baby’s gender have been passed down from generation to generation. And one reason they may stick around is that they’re often right; in fact, they’re right about half of the time.

You have a 50-50 chance of guessing your baby’s gender correctly no matter what means you use to formulate that guess. So if a threaded needle hung above your belly swings in circles (which traditionally means it’s a girl) and you have a girl, you might be tempted to believe that the needle method works. But in reality, it was probably just chance.

How a baby’s gender is determined

A baby's gender is determined by the father. Some sperm contain Y chromosomes, and some contain X chromosomes. If the father provides an X chromosome, the baby will be female, and if he supplies a Y chromosome, the baby will be male. Mothers can provide only an X chromosome, so they cannot influence their offsprings’ gender.

Each naturally occurring conception has an equal chance of being a boy or girl for two reasons:

  1. Sperm containing X chromosomes and sperm containing Y chromosomes are nearly equal in number.
  2. The chances that either type of sperm will fertilize an egg are roughly equal.

Changing your odds

If you are trying for a particular gender, you may have heard that you can influence whether you conceive a boy or girl. It’s been said that the following influences a baby’s gender:

  • The position during intercourse
  • Your diet
  • The lunar cycle
  • Gender patterns in the couple's families

Despite what you may have heard, though, none of the above has been confirmed to have any significant influence. In fact, there is relatively little a couple can do—through naturally occurring conception—to alter the chances of conceiving a boy or a girl.

However, the timing of intercourse can have an effect. Sperm carrying Y chromosomes apparently swim faster but do not survive as long as their X-carrying counterparts. So if a couple has intercourse right at the time of ovulation, the faster-swimming male sperm have an advantage, and a male baby is slightly more likely—but only very slightly. But if the timing is a day or two earlier, more of the male sperm die out, and a female baby is a bit more likely.

Even so, it is difficult (or, perhaps, impossible) to time ovulation with enough precision to alter the gender odds. And even if ovulation were accurately predicted, your odds of affecting gender are slim anyway.  Without the application of advanced (and sometimes invasive and expensive) technology, such as in vitro fertilization, the chances of conceiving a boy or a girl remain close to equal.

How can you tell your baby’s gender?

There are only three reliable ways to tell whether you’re having a boy or girl.

  1. Have an invasive procedure, such as amniocentesis or chorionic villus sampling (CVS). During these tests, cells are obtained from the fetus.  Chromosomal analysis of these cells can reveal much about the baby, including gender. However, these procedures are usually performed to detect potential health problems such as Down’s syndrome, spina bifida, or cystic fibrosis, not to find out a baby’s gender.
  2. Get an ultrasound.  By the 16th to 20th week of pregnancy, the ultrasound technician should be able to identify your baby’s sex. Occasionally, though, babies may be in a position where their genitals are hidden on the ultrasound picture. Some parents call that “being shy,” but whatever you call it, it can be annoying if you were hoping to find out the gender.
  3. Wait until the baby is born.

It seems increasingly rare that parents don’t know the gender of their baby prior to delivery.  Yet, tests to determine gender are not required for a routine pregnancy that’s going smoothly.  Apparently, most of us just can’t stand the suspense—and we’re not willing to count on the needle and thread method.