Let’s get straight to the point.

Of the various not-really-scientific-ways to “predict” the gender of your fetus, these 2 were the only ones I could find that are possibly based in truth.

• Fetal heart beat. Above 140 beats per minute: girl. Slower: boy.

• Morning sickness. Horrible: girl. Little or none: boy.

The proof is in the pudding.

Heart rate theory:
I found 2 studies that compared the rate of male and female fetuses, to try to prove this adage right or wrong. Unfortunately for our predicting powers, both studies did not find a correlation between the heart rate and the sex of the baby.

A study done in 1964, showing us this myth has been around for years, measured the fetal heart rate weekly during the last 2 months of gestation. The mean heart rate for females was higher than the mean heart rate for male fetuses, but the difference was not statistically significant. No convincing argument there.

Another study of fetal heart rate in early labor also did not provide any evidence. Even between brothers and sisters, no difference in heart rate was seen. I can tell you anecdotally, with twins of different sexes, I haven’t seen a difference either.

The origin of this myth is thought to be related to the scientific theory that heart rate correlates with size – the larger the size of an organism, the slower the heart beat. Humans have a heart rate of about 50 beats per minute, and rats have a heart rate of 250 beats per minute (yes, we are comparing rats and humans). Male babies are, in general, larger than female babies at time of birth. If you extrapolate the heart rate theory to this observation, males would have slower heart rates than females.
But … I can tell you this extrapolation doesn’t totally make sense. A normal fetal heart rate in early gestation is usually fast: 170 bpm. A rat is bigger than the cashew nut sized fetus, so why is the rat still faster? Doesn’t make sense.

I’m rating this one: not fact, not nonsense. Somewhere in between.

Morning sickness theory:
A large study from Sweden looked at women who were admitted to the hospital early in their pregnancy for hyperemesis gravidarum – the severe form of nausea and vomiting of pregnancy that hit mainstream when the already too skinny Duchess of Cambridge was hospitalized. Fifty-five percent of these admitted patients ended up with female offspring. Because the study included thousands of pregnancies, this 55% was statistically significant.
So yes, this theory does have some scientific basis. Finally!

Unfortunately though, I don’t think it’s enough evidence to be able to “predict” gender (remember, 45% of women sick enough to be hospitalized ended up with male offspring).

So we’re still out of luck. I think the magical prediction powers are losing street cred.

Studies cited:

Bernard, J. Prediction from human fetal measures. Child Development, 1964, 35, 1243-1248.
Petrif, Bruce and Sidney Segalowitz. Use of fetal heart rate, other perinatal and maternal factors as predictors of sex. Perceptual and Motor Skills, 1980, 50, 871-874.
Askling J, et al. Sickness in pregnancy and sex of child. Lancet. 354(9195):2053, 1999 Dec11