The gender of a fetus can be determined fairly accurately using only a small sample of maternal blood, according to an analysis published recently in JAMA. But it's only about 95% accurate after seven weeks and 99% accurate after 20 weeks. That's a far cry from the 99.9% accuracy by five weeks of pregnancy claimed by Acu-gen Biolabs, the makers of the now discredited Baby Gender Mentor, back in 2005.
Fetal gender determination before 20 weeks of pregnancy relies on analyzing the small amount of fetal DNA that appears in maternal blood. If Y-chromosomal material is present then the fetus must be a male (XY); if not, then the fetus must is a female (XX).
One note of caution; the fetal DNA in maternal blood must be amplified before testing because it is present in such small quantities in early pregnancy. The authors found that one of several DNA-amplification methods, called “real-time quantitative polymerase chain reaction” (RTQ-PCR), gave the most reliable results by 7 weeks.
Ultimately, DNA testing of maternal blood will probably supplant ultrasound (a.k.a. sonography) for gender testing, because ultrasound isn’t even very reliable until fairly late in gestation. However, DNA testing is still new and not yet reimbursed by insurers. Perhaps that will change with time.