Ultrasounds have a variety of purposes during pregnancy, but the use that often receives the most attention is its 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 choose to put off knowing the sex 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 baby's gender before delivery. They do so 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," Carr said. "It's the last great surprise left," he noted.
Increasingly, Carr said, couples have asked him to write down the baby's sex and place the answer in a sealed envelope. This is because some parents-to-be want to host a gender-reveal party for family and friends to share 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.
Lately, as more couples turn to in-vitro fertilization procedures to conceive a child, the issue of selecting the sex of a baby for purposes of "family balancing" rather than for medical reasons is controversial and raises ethical concerns.
Here six important facts about ultrasound screenings and sex determination, according to Carr, a professor of obstetrics and gynecology at the Warren Alpert Medical School of Brown University.
- A baby's gender is determined when the egg is fertilized.
- Whether a baby will be a boy or a girl 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 contribute either an X or a Y chromosome, depending on the sperm cell.
- The sex of the baby is determined by the sperm cell that 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 scan wasn't intended as an exam to find out the baby's gender; it was meant to image the developing fetus for other medical reasons, he said.
Although the test can be done at any point during pregnancy, women typically get one during the first trimester. This early ultrasound is often done to confirm a pregnancy, detect the fetal heartbeat and determine the due date, according to the March of Dimes.
A second ultrasound is usually done between the 18th and 22nd weeks of pregnancy to make sure that the baby is growing and developing properly. It's typically during the second ultrasound that parents can learn the sex of the baby.
The scan is also done 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, ultrasound can identify certain birth defects, such as Down syndrome and spinal abnormalities, and investigate pregnancy complications, including miscarriage, according to the March of Dimes.
It's a safe test for mother and baby
According to researchers at São Paulo Federal University, who published 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. Next, a probe called a transducer is moved over the woman's abdomen, which transmits sound waves that can produce images of the fetus as it develops inside the mother's womb.
There is no harm to the baby during the procedure, Carr said. And the only risks to the mother may come from lying flat on her back, which might make her feel dizzy, along with the discomfort of having a full bladder, he said. (Women may be asked to drink several glasses of water before an ultrasound 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.
After 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 that it is not in a breech, or feet-down position), and the 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, he 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. This screening, which is called the cell-free DNA test, is done at the eighth or ninth week of pregnancy. It may be used in women when 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 known 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 to do one, 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 may have limited medical training to interpret the scans, he noted.
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 the 18th week of pregnancy to find out the sex of a baby can feel like an eternity. To fill the void, people may turn to some of the following six old wives' tales to predict whether the fetus is a boy or girl.
Baby bump: 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," he said.
Food cravings: 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.
Fetal heart rate: There may be some truth to the idea that fetal heart rate could be a clue. Early in pregnancy, there is no difference in heart rate between the sexes, Carr said. But by the third trimester, a girl's heartbeat tends to be a little faster and a boy's a little slower, he said. Still, Carr cautioned that although researchers may find this association holds true over an average of 1,000 babies studied, an individual baby boy could still have a faster heartbeat, and an individual baby girl could have a slower one.
Morning sickness: Folk wisdom has linked experiencing severe morning sickness with having a girl, and this idea may have some science to back it up. 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.
The Drano test: For this urban legend, 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.
Ring test: To try this old favorite, a woman ties her wedding band to a string and hangs it over her pregnant belly to guess the baby's gender. If the ring swings back and forth, the baby is believed to be a boy. If it swings in a circle, the child is thought to be a girl. "It's fun, but it isn't science," Carr said, chuckling.