The standard way for couples to find out the gender of their babies these days is during a routine ultrasound during the second trimester. The prediction is based on ability of the ultrasonography equipment and the ultrasonographer to distinguish a developing penis from a developing clitoris, as both of these organs develop from the same embryonic structure (see more about this here). Generally, this distinction becomes trustable around gestational age 18 to 20 weeks (18-20 weeks from the last menstrual period), or sometimes as early as 16 weeks. It’s also fairly common for couples to find out the gender earlier if there is a risk factor that warrants a procedure that can provide cells. One procedure is amniocentesis in which your obstetrician uses needle to extract fluid from amniotic sac within your womb, fluid that contains some cells whose genome (set of genetic sequences) matches that of the fetus. Often performed as early as 14 weeks gestation, and sometimes as early as 11 weeks, amniocentesis is indicated if you are at least 35 years old, if you have any of several risk factors, namely:
- History of a chromosomal condition, or a neural tube defect (layers of tissue around the brain or spinal cord do not completely close), in a previous pregnancy
- Family history of a genetic condition
- Abnormal results on a prenatal ultrasound or blood test
Alternatively, there is another procedure called chorionic villus sampling (CVS) may be indicated for women with risk factors, sometimes as early as 10 weeks gestation. Rather than drawing amniotic fluid, in CVS the doctor draws samples of the placenta, providing cells that also have the same genome as those of the fetus. With cells obtained in amniocentesis or CVS, laboratory personnel perform what’s called a karyotype, a chromosomal analysis. This reveals chromosomal abnormalities, if present, but with or without abnormalities it also reveals whether each cell contains two X chromosomes meaning that the fetus is a girl, or one X and one Y chromosome, in which case it is a male fetus.
For those who do not have risk factors, the above tests are not indicated, and your insurance won’t pay for them. Furthermore, even if you wish to pay yourself for amniocentesis or CVS with gender analysis, your obstetrician is not likely to sign off on it if you have no risk factors, because there is a small, but real, risk that the needle could harm the fetus (really the risk is low these days, since the procedure is performed with ultrasound guidance, but its real enough to worry obstetricians and their attorneys). Consequently, many couples are on the lookout for tests that are not dangerous, that are relatively inexpensive, and that can reveal the gender of their future children as early, or earlier than CVS.
In recent years, there has been a flood of take home, do-it-yourself, tests available at drugstores that are marketing with claims of accurate gender prediction as early as 6 weeks gestation. Some of these very early gender tests are based on detection of male hormones in maternal urine, male DNA in blood samples, or other tactics, but claims about the accuracy often have not been reliable.
Using the tactic of detection of male DNA, however, accuracy rises significantly by a gestational age of 9 weeks, slightly earlier than when CVS can be performed. A popular DIY gender kit in the United States that works on the male DNA principle is called Sneak Peek, which can be reliable so long as you follow the rules, which include not taking the test until you reach nine weeks gestation, and keeping all males away from the blood sample that you obtain by pricking your finger. Importantly, this means you must not have your male partner help you with the finger prick, nor even in packaging the sample into an envelope.
So how does it work? You cells have 46 chromosomes -22 pair of autosomes, which are the same in males and females, and one pair of sex chromosomes. In females, both sex chromosomes are X chromosomes (one you inherited from your mother, the other from your father), cells of a male have just one X chromosome (inherited from the male’s mother), plus a Y chromosome, inherited from the male’s father. In a male, the X and Y chromosome comprise the 23rd pair, because a section of the Y chromosome is homologous with portions of the X chromosome, meaning that it holds the same genes. However, there also are non-homologous parts of the Y chromosome, genetic sequences that are unique to the Y chromosome, and thus unique to males. Women, girls, and female fetuses do not have these Y sequences in their cells. Consequently, you, as a woman, normally do not have these Y sequences in your blood stream. However, although the blood vessels of the placenta maintain a barrier that isolates the embryo or fetus from your blood stream, such that fetal DNA should not enter your blood, the isolation is not perfect. Small quantities cells of the placenta, which have the same genome as cells of the fetus, or their contents, can enter your bloodstream. Furthermore, small breakages of blood vessels in the placenta allow DNA from cells of the fetus itself to enter your blood.
If you are carrying one or more female fetuses, or if you have not had a male fetus inside of you for the past few months or more (you are at least a few months beyond delivering a boy), your blood will not have DNA sequences of the non-homologous portion of the Y chromosome in quantities that can be detected by the gender DNA test. On the other hand, if you are carrying a male fetus, there will be a certain amount of Y chromosome DNA in your blood, and this can be detected in your blood.
If you have a multiple pregnancy, the key to what the test can tell you is that it is looking for Y chromosome DNA. Thus, it can tell you whether or not there is at least one male among the fetal. If the test is negative for male DNA, and you have twins, then you have two girls, but it is positive for male DNA you’ll only know that at least one of the two is a boy.
There are several ways that you can get a false positive result (a false indication that you are carrying a boy). If you received blood products (packed red blood cells or some other donated blood product) in the past few months, for instance, it’s possible that this could give you a false positive result for male DNA, even though red blood cells do not have nuclei and chromosomes. The reason is because the test is extremely sensitive and so only minute amounts of male DNA need be present. A much more common reason for a false positive, however, is that somewhere a long the way, from your blood to the lab where the sample is tested, a male contaminates the sample. Your male partner is a likely suspect.