11 Big Fat Pregnancy Myths

"It's actually more like nine-and-a-half months," said Dr. Joanne Stone, an obstetrician at Mount Sinai School of Medicine in New York City and co-author of "Pregnancy for Dummies" (For Dummies, 2009).

To make matters more confusing, doctors typically measure pregnancy's length as 40 weeks, counting from a woman's last period. Women usually become fertile 10 to 16 days after their period starts, so from this method of counting, the first two weeks of every pregnancy take place before a woman has conceived.

Dads-to-be have two types of sperm: those that make baby boys and those that make baby girls. Some say that the body of a mom-to-be is more hospitable to boy sperm or girl sperm during certain times in her cycle. And so, the theories go, when a couple has sex can determine whether they conceive a Junior or a Bubette.

While a few small studies have supported timing methods for sex selection, larger, more recent studies have failed to find any correlation between the day within the fertile window a couple has sex and the gender of the baby, explained Dr. Rachel Vreeman, co-author of "Don't Swallow Your Gum! Myths, Half-Truths, and Outright Lies about Your Body and Health" (St. Martin's Griffin, 2009).

The only way you can reliably choose the sex of your baby is with the help of technology.

Time to break it to grandma: Basketball-shaped bellies do not forecast boys. In a study of 104 women, published in the journal Birth in 1999, no correlations were found between a pregnant belly's size or shape and the baby's gender.

Similarly, predictions using the Chinese lunar calendar, the fetal heart rate and the Drano test (where the woman's urine is mixed with the de-clogging liquid and the resulting color allegedly reveals the baby's gender) were examined by physicians in Vancouver in 1999. None of them were reliable.

Neither is women's intuition dependably accurate, according to a 1996 article in the Journal of Reproductive Medicine. Of 110 women who professed having a strong sense of the baby's gender, roughly half were right the same number expected to be right if they were just guessing.

In all, scientists have found most folklore-based methods of predicting a baby's sex have the same odds as flipping a coin. "And 50/50 odds are not that bad," Vreeman said.

There may be a case for the predictive value of extreme morning sickness. Excessive, unrelenting morning sickness also known as hyperemesis gravidarum is slightly more correlated with having girls, Vreeman said.