Some prenatal gender tests that use mom's blood are very accurate at determining baby's sex, a new study finds. But curious parents-to-be should be wary of online marketers that claim to be able to figure out fetal gender using just a woman's urine. 
 
 New research to be published Aug. 10 in the Journal of the American Medical Association finds that after seven weeks into a pregnancy, tests that analyze mom's blood for fetal DNA can correctly identify a male fetus 95.4 percent of the time and a female fetus 98.6 percent of the time on average. In comparison, tests that analyzed DNA from urine instead of blood were only accurate 41 percent of the time, said study researcher Diana Bianchi, a reproductive geneticist at Tufts University School of Medicine. 
 
 "It was worse than flipping a coin," Bianchi told LiveScience. 
 
 Why baby's sex matters 
 
 Ultrasound imaging can sometimes reveal the sex of a baby as early as 11 weeks into pregnancy, though the results are wrong as much as 40 percent of the time. Most pregnant women in the United States get an ultrasound between 18 and 22 weeks of pregnancy that looks for fetal anomalies. At that point, the fetus' sex can be determined with high accuracy. [5 Myths About Women's Bodies] 
 
 Some people don't like to wait that long. Chelsea Gladden, who blogs at breezymama.com, told LiveScience that she and her "swollen ankles" needed the excitement of finding out her baby's sex about halfway through the pregnancy, but said she would have found out earlier if she could have. 
 
 "I was definitely consumed with finding out," Gladden said. 
 
 But curiosity isn't the only reason for earlier gender testing. Certain genetic disorders are linked to the X chromosome, so they overwhelmingly affect males, whose XY sex chromosomes mean they lack the "backup" X that women have. Families at risk for these disorders can now opt to have amniocentesis, in which the fluid that cushions the fetus in the womb is extracted and tested, or a procedure called chorionic villus sampling, both of which carry a small risk of miscarriage. 
 
 A non-invasive blood test would cut down on such testing by 50 percent because moms carrying female babies wouldn't need to worry, said Bianchi. Bianchi is on the advisory board and holds stock options in the biotechnology company Verinata Health, Inc., which has the goal of developing non-invasive fetal abnormality tests. 
 
 Another disorder, congenital adrenal hyperplasia, or CAH, disrupts hormone balance, resulting in a female fetus taking on masculine traits. Moms carrying fetuses with CAH take steroids during their pregnancies, which can have unpleasant side effects. If fetal sex were known earlier, Bianchi said, the moms carrying male fetuses with CAH would be able to skip the steroids. 
 
 Chromosome-based diagnoses of gender are also important to parents whose children are born with ambiguous genitalia. If an ultrasound reveals genitals that could be male or female, Bianchi said, knowing the baby is XX or XY can give parents a road map for what gender to raise the child. [The Truth About Genderless Babies] 
 
 It's a boy! (or girl) 
 
 Blood tests for fetal gender aren't available clinically in the United States, Bianchi said, though they are used in Europe for diagnosis in high-risk pregnancies. A number of companies do sell blood and urine tests of fetal sex to parents online for several hundred dollars. 
 
 Bianchi and her colleagues combed through the scientific literature to find studies on those blood and urine tests that use fetal DNA from mom's blood to identify sex. Only males have Y chromosomes, so if Y chromosomes are found in mom's blood, she's likely carrying a baby boy. If no Y chromosomes are found, she's probably expecting a daughter. 
 
 After excluding studies that lacked data or were too small, Bianchi and her team came up with 57 studies of prenatal sex tests to analyze. They found that urine tests were extremely unreliable, possibly because by the time fetal DNA is filtered from the blood into the urine, it's been broken down. 
 
 Blood tests, on the other hand, revealed fetal DNA quite early. Before seven weeks, blood tests correctly identified male fetuses only 74.5 percent of the time. After seven weeks, however, accuracy went up. Tests conducted between seven and 20 weeks of pregnancy accurately identified baby boys about 95 percent of the time and baby girls about 99 percent of the time. After 20 weeks, these test were extremely accurate, pegging boys as boys 99 percent of the time and girls as girls 99.6 percent of the time. 
 
 Parents of at-risk pregnancies should talk with their doctors, Bianchi said, because blood tests could help prevent more invasive procedures down the road. Bianchi said she sends samples to the U.K. when she really needs a test done. 
 
 But for moms and dads who just want to know what color to paint the nursery, Bianchi recommends patience. 
 
 "Currently, they would have to go to the Internet, and I would say that they should be wary," she said. "There is not a whole lot of transparency in those methods or the actual performance results." 
 
 The study was funded by the National Human Genome Research Institute. 
 
 You can follow LiveScience senior writer Stephanie Pappas on Twitter @sipappas. Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on Facebook. 

 A simple blood test that can determine a baby’s sex as early as seven weeks into pregnancy is highly accurate if used correctly, a finding that experts say is likely to lead to more widespread use by parents concerned about gender-linked diseases, those who are merely curious and people considering the more ethically controversial step of selecting the sex of their children. 
 
 The appeal of the test, which analyzes fetal DNA found in the mother’s blood, is that it can establish sex weeks earlier than other options, like ultrasound, and is noninvasive, unlike amniocentesis and other procedures that carry small risks of miscarriage. The finding came in a study published online Tuesday in The Journal of the American Medical Association. 
 
 The tests have been available to consumers in drugstore chains and online for a few years, but their use has been limited, partly because their accuracy was unclear. One company, which guaranteed 99.9 percent accuracy as early as five weeks into pregnancy, filed for bankruptcy after a lawsuit by scores of women whose tests showed the opposite sex of the baby they ended up having. 
 
 European doctors now routinely use the tests to help expectant parents whose offspring are at risk for rare gender-linked disorders determine whether they need invasive and costly genetic testing. For example, Duchenne muscular dystrophy affects boys, but if the fetus is not the at-risk sex, such tests are unnecessary. But doctors in the United States generally have not prescribed the tests because they are unregulated and medical labs are not yet federally certified to use them. 
 
 That and other aspects of the pregnancy landscape could change as a result of the new study. The journal study analyzed reams of research on fetal DNA tests — 57 studies involving about 6,500 pregnancies — and found that carefully conducted tests could determine sex with accuracy of 95 percent at 7 weeks to 99 percent at 20 weeks. 
 
 The study “has wide-reaching implications,” said Dr. Louise Wilkins-Haug, director for maternal-fetal medicine and reproductive genetics at Brigham and Women’s Hospital in Boston, who was not involved in the research. “Individuals need to be careful” to ensure that companies use rigorous laboratory procedures and support accuracy claims with data, she added. 
 
 One potential worry is that women might abort fetuses of an undesired sex. Several companies do not sell tests in China or India, where boys are prized over girls and fetuses found to be female have been aborted. While sex selection is not considered a widespread objective in the United States, companies say that occasionally customers expressed that interest, and have been denied the test. A recent study of third pregnancies in the journal Prenatal Diagnosis found that in some Asian-American groups, more boys than girls are born in ratios that are “strongly suggesting prenatal sex selection,” the authors said. 
 
 At least one company, Consumer Genetics, which sells the Pink or Blue test, requires customers to sign a waiver saying they are not using the test for that purpose. “We don’t want this technology to be used as a method of gender selection,” said the company’s executive vice president, Terry Carmichael. Sex-determination tests are part of a new frontier of fetal DNA testing, which can be used to determine paternity and blood type, and is being used to develop early screening tests for genetic diseases or disorders like Down syndrome. 
 
 The new study found that to be reliable, the sex-determination tests had to be performed after at least seven weeks of gestation. Most tests that were highly accurate were conducted on a mother’s blood, not urine. And certain rigorous laboratory procedures had to be followed. For the blood tests, women prick their fingers and send blood samples to labs. If the Y chromosome is detected, the fetus is male. Absence of a Y chromosome would probably mean the fetus is female, but could mean that fetal DNA was not found in that sample. 
 
 The tests are not regulated by the Food and Drug Administration because they are not used for medical purposes, a spokeswoman said, but the agency is investigating the explosion of home genetic tests like these and genome-sequencing kits. 
 
 Dr. Diana Bianchi, executive director of the Mother Infant Research Institute at Tufts Medical Center in Boston and the lead author of the sex-determination report, said, “A very important aspect of the study is how this advances prenatal care.” 
 
 But there are potential concerns too, she said, including that women may spend more than $250 for the tests when they don’t have insurance for prenatal care. A typical blood test like Pink or Blue, for example, costs $25 for the kit. Lab fees and shipping costs, which vary, bring the total expense to $265 to $330. 
 
 Dr. Bianchi is conducting another study to “try to find out why people are buying these things and what are the consequences,” she said. “It’s very important to educate health care providers that pregnant women are buying these tests.” 
 
 Another type of test not studied by the researchers has become popular because it is cheaper and can be done at home. These tests analyze hormones in women’s urine, a method that several experts said has not been studied as rigorously as DNA. Rebecca Griffin, a founder of the biggest seller, Intelligender, said two independent studies found it 90 percent accurate at 10 weeks. 
 
 Another company, TrovaGene, has developed a DNA test using urine, which, according to Gabriele Cerrone, TrovaGene’s co-founder and director, is 95 percent accurate at predicting boys at seven weeks, and 88 percent accurate at predicting girls. TrovaGene is also developing a test for Down syndrome. 
 
 Most DNA tests on the market use blood. 
 
 Raylene Lewis, 34, of College Station, Tex., had a frustrating experience with a now-defunct company, Acu-Gen, which guaranteed 99.9 percent accuracy with its blood tests. In 2005, she was told she was having a boy, and she chose a name, bought boy clothes and told everyone. When an ultrasound revealed she was carrying a girl, “I was absolutely shocked,“ she said. She was not unhappy, she said, but “it was like the baby boy disappeared.” 
 
 When the Lewises complained to Acu-Gen’s president, they were told, “We are very sure that genetically you are having a male,” she said, reading a transcript of the conversation, which they recorded. The official suggested that the baby might be “what a baby girl looks like on the outside,” but that “we’re giving you the results on the inside.” 
 
 Ms. Lewis, who sued Acu-Gen, said she ultimately received a refund. Lawyers for Acu-Gen could not be reached for comment. 
 
 The Pink or Blue test, which claims 95 percent accuracy at seven weeks and gives refunds for wrong results, appears to meet the standards described in the new study. 
 
 Chelsea Wallace, 23, of Okeechobee, Fla., was thrilled early this year when the test she took at seven weeks said she was having a boy, a result confirmed weeks later by ultrasound. Ms. Wallace, who has a 3-year-old daughter, said she also would have been happy with a girl, but that since she and her fiancé could not agree on a boy’s name, finding out early gave them time to choose the name Layton. It also helped her plan, she said. 
 
 “As soon as I found out I was pregnant,” said Ms. Wallace, whose son is due in September, “I wanted to know what I was having.”
– Blood tests that promise to determine a baby's sex just seven weeks into pregnancy are highly accurate if used properly, a new study finds. Researchers found that the tests, which analyze fetal DNA in the mother's blood, are 95% accurate at seven weeks and 99% accurate at 20 weeks, the New York Times reports. Tests that analyzed urine instead of DNA, however, were accurate just 41% of the time—a prediction rate "worse than flipping a coin," a researcher tells LiveScience. The blood tests are sold online and in drug stores and are widely used in European hospitals—but not American ones. The technology to accurately predict the sex of the fetus so early on will help families worried about gender-linked diseases, experts say. But it raises fears that the tests will be widely used by parents wishing to choose the sex of their children, especially in China and India, where the boy-girl ratio is already greatly distorted because of sex-selective abortions.