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Ultrasounds have a variety of purposes during pregnancy, but the use that often receives the most attention is the technology's ability to reveal the sex of the baby.
Some parents-to-be can't wait to find out whether they're having a boy or a girl, while others just choose to hold off until birth. Either way, a sonogram — the grainy, black-and-white image that results from an ultrasound scan — will be baby's earliest picture and a couple's first chance to see the developing fetus.
Ultrasound uses high-frequency sound waves to produce an image on a screen of the baby in the mother's uterus. The scans are typically done twice during pregnancy, but the one done between 18 and 22 weeks is when the sonographer (ultrasound technician) might identify the gender of the baby, if parents want to know.
Expectant parents who want their child's sex to remain a secret until birth are in the minority, said Dr. Stephen Carr, director of the Prenatal Diagnosis Center and of maternal-fetal medicine diagnostic imaging at Women & Infants Hospital of Rhode Island in Providence. He said about 85 percent of couples want to find out the gender before the delivery, and for several reasons: to know how to paint the nursery, pick a name or satisfy their curiosities about the family composition.
However, "more and more people are telling us they want to wait until the baby arrives to find out the sex," he said. "It's the last great surprise left."
Increasingly, Carr said, couples have asked him to write down the baby's sex and place the answer in a sealed envelope, because some parents-to-be want to host a gender-reveal party for family and friends to share and celebrate the news.
Beyond ending the guessing game, there are medical reasons why mothers and fathers may want to learn the sex of their baby from an ultrasound. Sometimes knowing the gender can help parents make informed decisions about the pregnancy itself, such as in the case of sex-specific diseases, Carr told Live Science.
One example of this is congenital adrenal hyperplasia, a genetic condition in which baby girls may have genitals that appear more masculine than feminine, according to the Mayo Clinic.
Carr, also a professor of obstetrics and gynecology at the Warren Alpert Medical School of Brown University, discussed six key points about ultrasound screenings and sex determination.
A baby's gender is determined when the egg is fertilized
A baby's gender is determined at the time of conception, long before most women even realize they are pregnant.
Both the egg from the mother and the sperm from the father carry sex chromosomes. The egg always contributes an X chromosome, and the sperm can bring either an X or a Y chromosome, depending on the sperm cell.
The sex of the baby is determined by which sperm cell fertilizes the egg first. If the sperm is carrying an X chromosome, the baby will be a girl. If the sperm is carrying a Y chromosome, the baby will be a boy.
Ultrasounds were not designed for sex prediction
U.S. hospitals have used ultrasounds since the late '70s and early '80s, Carr said. But the common prenatal exam wasn't intended as a reason to find out the baby's gender; it was meant to image the developing fetus for other reasons, he said.
Although the test can be done at any time during a pregnancy, women typically get one during the first trimester. This early ultrasound is often done to confirm a pregnancy, detect the fetal heartbeat and to determine the due date, according to the March of Dimes.
A second ultrasound is usually done 18 to 22 weeks into pregnancy to make sure the baby is growing and developing properly. This is often when parents can learn the sex of the baby.
The scan is also used to see if a woman is having more than one baby, as well as to determine the location of the placenta and umbilical cord. In addition, it can identify certain birth defects, such as Down syndrome and spinal abnormalities, and investigate pregnancy complications, including miscarriage, reports the March of Dimes.
It's a safe test for mother and baby
According to researchers at São Paulo Federal Universityin a 2009 article in the journal Ultrasound in Obstetrics & Gynecology, ultrasound is a safe prenatal test. It uses sound energy and not radiation, such as X-rays, to generate images of the fetus.
During a transabdominal ultrasound, a pregnant woman lies on her back while a clear gel is spread on her belly, according to Johns Hopkins Medicine. A probe called a transducer is moved over the abdomen, transmitting sound waves that produce images of the baby inside the mother's womb.
There is no harm to the baby during the procedure, and the only risks to the mother may come from lying flat on her back, which might make her feel dizzy, or the discomfort of having a full bladder, Carr said. (Women may be asked to drink several glasses of water before the scan because a filled bladder helps give clearer images.)
Gender determination is usually highly accurate
Gender predictions made by ultrasound have an accuracy rate "north of 90 percent," Carr said. But mistakes can be made when determining gender because it depends on the clarity of the images and the skills of the person interpreting them.
Until the 14th week of pregnancy, baby boys and girls look exactly the same on ultrasound, Carr said. Beyond this point, noticeable anatomical differences in the genitals can show up on the scan.
Around 18 weeks of pregnancy and beyond, Carr said that ultrasounds have pretty good reliability for gender prediction if the baby is in a good position in the mother's uterus (meaning it is not in a breech, or feet-down position), and its legs are far enough apart that there is good visibility between them.
"Gender-telling is not exotic," Carr said. When a sonographer looks between the legs, if it's "an outie," it's a boy, Carr explained.
A blood test for pregnant women that's been around for about three years can also determine the sex of the fetus with 98 to 99 percent accuracy, Carr said. Done at the eighth or ninth week of pregnancy, the screening is called the cell-free DNA test. It may be used in women where there is an increased risk of chromosomal abnormalities, such as in older mothers, he said.
Be wary of keepsake ultrasounds
Carr said that he understands the psychology of expectant parents wanting to see an image of their baby. However, he doesn't endorse so-called "bonding scans," which are also referred to as recreational or keepsake ultrasounds. These scans are done to produce keepsake pictures or videos, and not for medical reasons.
Ultrasounds should be used as a diagnostic tool when there's a medical reason, Carr said. The procedure is tightly regulated when it occurs in a hospital or medical clinic, he added.
That's generally not the case for commercial places doing keepsake images: There is no regulation of ultrasound facilities outside of a medical setting, so their quality can vary wildly, Carr said, and the technicians there may have limited medical training to interpret the scans.
The American College of Obstetricians and Gynecologists and the American Institute of Ultrasound in Medicine issued a joint statement discouraging the use of prenatal ultrasounds for nonmedical reasons.
Old wives' tales are fun but generally not reliable for predicting sex
For some people, waiting until at least the 18th week of pregnancy to know the sex of a baby can feel like an eternity. Perhaps that's why people continue to believe so many old wives' tales as prediction methods to fill the void.
One popular belief is that if a woman is carrying the baby high, she is supposedly having a girl, while carrying the baby low means it's a boy. "Carrying high or low is a function of the mother's abdominal wall muscle tone and the baby's position," Carr said. "It has no influence on gender."
Another theory holds that a mother's food cravings during pregnancy may reveal the baby's sex, with sweet cravings signifying a girl and cravings for salty, sour or odd foods linked with a boy. "This has no basis in physiology," Carr said.
But there may be a little bit of truth to the idea that fetal heart rate could be a tip-off. Carr said that early in pregnancy, there is no difference in heart rate between the sexes, but by the third trimester, a girl's heartbeat tends to be a little faster and a boy's a little slower. Still, he cautioned that although research studies may find this association over an average of 1,000 babies, an individual baby boy could have a faster heartbeat, and an individual baby girl could have a slower one.
Folk wisdom has linked experiencing severe morning sickness with having a girl, and this tale may have some science behind it. Women carrying girls have higher levels of the pregnancy hormone hCG (human chorionic gonadotropin), and these higher levels are associated with an increased risk of severe morning sickness, Carr said. But he warned there's not a hard and fast relationship between morning sickness and fetal sex.
Another urban legend is "the Drano test." For this one, a woman combines some of her first morning urine with the liquid drain cleaner. If the color turns green, the baby is said to be a girl; if it's blue, a boy may be on the way. Unfortunately, "there's nothing to this idea, and Drano is really caustic," Carr pointed out.
The ring test is another suggested way to guess the baby's gender. For this old favorite, a woman ties her wedding band to a string or strand of long hair and hangs it over her pregnant belly. If the ring swings back and forth, the baby is believed to be a boy. If the jewelry swings in a circle, the child is thought to be a girl. "It's fun, but it isn't science," Carr said, chuckling.
- Johns Hopkins Medicine explains: What Is a Fetal Ultrasound?
- See a slide show explaining fetal ultrasound at the Mayo Clinic.