A baby's gender is decided at the moment of conception, according to the website KidsHealth. All of a mother's eggs have an X -- or female -- chromosome, and each of the father's sperm carries either an X or a Y chromosome. A baby's sex is determined by the chromosome of the sperm that gets to the egg first. If the sperm has an X chromosome, the baby now has two X chromosomes and she is a girl. If it has a Y chromosome, the Y chromosome prevails and he is a boy.
Theory
The myth goes that if your baby has a fetal heart rate above 140 beats per minute, she's a girl. Anything less, and he's a boy. Notably, no version of this old wives' tale tells you during what week of your pregnancy this is supposed to occur. Most babies have a heart rate over 140 beats per minute by seven to eight weeks after their mothers' last menstrual period, according to the website FetalSure.
History
FetalSure indicates that the idea of your baby's heartbeat predicting his gender might have gotten started because girls have higher metabolism than boys. And doctors did accept the idea that heartbeat was an indication of gender at one point, according to the website Pregnancy-Info, but the concept has now been labeled a myth.
Alternatives
An ultrasound after your twentieth week can give you a reliable idea of what you're carrying. It will probably be about 97 percent accurate. If you need to have an amniocentesis or chorionic villus sampling done, you can be virtually positive about the gender of your baby. Or just trust your instincts: Pregnancy-Info cites a study in which 71 percent of mothers correctly guessed the gender of their baby.
Research
FetalSure indicates that no research has borne out the premise that girls' hearts beat faster than boys in the womb. Terry J. DuBose, director of the diagnostic medical sonography program at the University of Arkansas, took part in an informal study into the issue in 1984. DuBose says that results indicated a difference of no more than a quarter of a heartbeat between girls and boys. There was no marked change between the rates at any point of the pregnancies of the women studied.
The Reality
Girls do have a faster heart rate than boys, according to Pregnancy-Info, but not until after they are born. Your baby's heart starts beating approximately 22 days after conception, according to FetalSure. In the beginning, it will match yours, which should be between 80 and 85 beats per minute. Each day, it will pick up its pace by a little more than three beats. By the ninth week after your last menstrual period, your baby's heartbeat will be in the neighborhood of 155 to 195 beats per minute -- so if the old wives' tale is to be believed, all babies would be girls.
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