Pregnant woman sitting // Pregnant woman sitting - © Erik Isakson | Getty Images(© Erik Isakson | Getty Images)

Pregnancy is a unique time with unique challenges; it's understandable to want to do all you can to deliver a healthy baby—but how do you decide what's best? Your own health care professionals can be an important resource, but what about things you read or hear between visits? You may not want to be calling your doctor to double-check everything.

Even for those things for which no answer is known, one thing is for sure: you'll get advice about it. No matter how popular, advice that is clearly wrong is worth ignoring. For the list of myths below, I've searched the web, asked readers of Aetna InteliHealth, and surveyed my friends and family. As expected, the number of opinions about what pregnant women should and should not do was exceeded only by the confidence placed in these rules and prohibitions. Here's a sampling of what I heard:

12 pregnancy myths

1. You can determine your baby's gender by the position of sexual intercourse and you can predict it by how you are carrying.

The gender of your baby is determined by the father; more specifically, if sperm carrying an X-chromosome fertilizes the egg, the baby will be a girl and if the sperm carrying a Y-chromosome fertilizes the egg, a boy will result. The position of intercourse has no clear effect on which type of sperm (X or Y) successfully fertilizes the egg.

The baby's size and position determine how one "carries" their baby. For example, during late pregnancy, the baby's head "drops" lower into the pelvis in preparation for delivery; that will make it seem that the mother is carrying "low," but that happens for male and female babies. A mother-to-be may be "carrying high" simply because the baby is large, not because of its gender.

2. The worse the morning sickness, the more likely it is you're having a girl.

Most experts believe this is truly a myth. However, several studies have found that among women with severe morning sickness bad enough to require admission to the hospital, slightly more than half (53% to 56%) delivered girls, so perhaps there is some truth to this idea. Then again, even among those with the worst morning sickness, the male and female offspring were nearly 50-50 and whether this applies to milder cases is unknown.

No one knows exactly why morning sickness (also known as hyperemesis gravidarum) occurs, though it has long been thought to relate to elevated hormone levels (including progesterone, estrogen and/or human chorionic gonadotropin, or HCG). Which hormone, if any of these, is most important, remains a matter of speculation, and several other theories have been proposed such as zinc deficiency, genetic factors and psychological factors. At least one study found that women carrying a female fetus had higher HCG levels than with a male fetus. If true, that could explain the connection.

3. If you raise your arms above your head while pregnant (as when you are hanging up clothes on a clothesline), the baby will get the cord wrapped around its neck.

Up to 25 percent of fetuses have the umbilical cord wrapped around the neck; it is the baby's activity in the womb and, perhaps, bad luck, not the mother's activities during pregnancy that determine whether the cord is wrapped around the baby's neck. Another important risk factor is a long umbilical cord, but, again, that may have more to do with fetal activity and is not something a pregnant woman can prevent. The good news is that nearly all of these babies develop normally and are successfully delivered.

4. If you get a lot of heartburn during pregnancy, your baby will have a lot of hair.

Heartburn is common during pregnancy because, as the stomach is pushed higher by the growing baby and the expanding uterus, it becomes easier for acid to move backwards (or "reflux") from the stomach into the lower esophagus. Acid is irritating to the esophagus and causes the discomfort we know as heartburn. Remaining upright after eating, sleeping with the head of the bed at a slight elevation, and antacid medications (many of which are considered safe during pregnancy) can be helpful. Check with your obstetrician before taking any medications during pregnancy, including over-the-counter remedies.

5. Avoid sleeping on your back or, always sleep on your left side.

During the later stages of pregnancy, the uterus and baby may be large enough to press on the large vein, the inferior vena cava, and reduce flow of blood from the lower body (and uterus) back to the heart. But this tends to matter only in certain circumstances such as prolonged labor, if blood pressure is high, if the kidneys are not functioning properly, or if there is a problem with fetal development. In those situations, lying on the left side may be somewhat helpful, but for normal, healthy women in the midst of a routine, successful pregnancy, the best position for sleeping is the one that's most comfortable.