Old wives' tales concerning babies abound, and perhaps none are more enduring than those predicting the sex of an unborn child.
One old maxim holds that a baby bump resting high and tight on a woman's belly foretells the birth of a boy, while a baby bump that sits low and wide across the mom-to-be's abdomen is a sure sign of a girl.
But people are looking in the wrong place, said science journalist Jena Pincott, who claims it's the shape of a woman's breasts — not her belly bump — that reveals the sex of the child.
In her new book, "Do Chocolate Lovers Have Sweeter Babies?" (Souvenir Press, 2013), Pincott states that moms carrying girls will develop larger breasts than moms carrying boys, the Daily Mail reports. [8 Ways a Woman's Body Changes During Pregnancy]
Pincott said in her book that the breasts of a woman carrying a girl will increase about 3.2 inches (8 centimeters), while those of a woman carrying a boy will grow only 2.5 inches (6.3 cm). She claimed the difference is due to the amount of testosterone that a male fetus creates — the hormone dampens the growth of an expecting mother's breasts.
The father's sperm determines the sex of his offspring: While a woman always provides an X chromosome to a child in her egg, the sperm that fertilizes her egg provides either a second X chromosome for a girl (XX), or a Y chromosome for a boy (XY). Worldwide, there are about 107 males conceived for every 100 females conceived, due to parental ages, stress, the stage in the mother's ovulation cycle and other factors.
The most accurate way to tell the sex of a developing fetus remains amniocentesis, which is 100 percent accurate but has a small risk of miscarriage. An ultrasound is safer, but can be inaccurate at determining a baby's sex if performed before 18 weeks of pregnancy.
A 2011 study from the Journal of the American Medical Association revealed that tests analyzing a mother's blood for fetal DNA after seven weeks can correctly identify a male fetus about 95 percent of the time and a female fetus about 98 percent of the time.
Urine tests to reveal the sex of a baby, however, are notoriously inaccurate and may be no better than flipping a coin, according to medical experts.
Email Marc Lallanilla or follow him @MarcLallanilla. Follow LiveScience on Twitter @livescience. We're also on Facebook & Google+. Original article on LiveScience.com.