The sex of a child is determined at fertilization. Females have two X chromosomes (XX), and males have an X and a Y chromosome (XY).
Each egg contains one X chromosome; males produce sperm with either an X or a Y chromosome (both types are formed in equal numbers in males).
The baby’s sex is determined by whether the egg was fertilized with the X or Y chromosome. If it was an X sperm, the baby will have female reproductive organs (XX). If a Y sperm fertilized the egg, the baby will have male reproductive organs (XY).
The widely held belief that X sperm are generally slower than Y sperm has never been confirmed. The belief started in the 1960s – and seemed true in various experiments – until computer assisted sperm analysis indicated there are no differences between the two.
In both male and female embryos, the same group of cells has the potential to develop into either the male or female gonads (organs that produce ovaries or testes). The early gonads begin to develop at 6 to 7 weeks of pregnancy.
Between 7 and 8 weeks of pregnancy, all embryos have both Wolffian (male) and Müllerian (female) ducts. One set of these ducts will fully develop based on the sperm type that fertilized the egg, while the other duct will degrade. Therefore, until 8 weeks of pregnancy, all embryos are sexually "indifferent".
At approximately 8 to 10 weeks of pregnancy, the sex that was determined at fertilization begins to externally develop and is fully distinct by 14 weeks of pregnancy and can be identified via ultrasound.
Around 8 weeks of pregnancy, a gene on the Y chromosome tells the fetal tissue that will form the sex organs to become the testes. The testes, in turn, produce testosterone to further male reproductive development, and the Müllerian ducts degrade.
Without this hormonal trigger to begin male reproductive development, the Müllerian duct remains, and female reproductive organs develop (and the Wolffian ducts degrade). Therefore, without the gene on the Y chromosome to start the process, and no hormonal trigger, the baby develops as female (which is why female is sometimes considered the fetal "default" sex).
If female, the ovaries start to develop at 14 weeks; estrogens produced by the mother’s ovaries and the placenta further stimulate development of the fetal uterus and vagina. Additionally, all oocytes ("eggs") a female will have are present by five months of gestation but remain dormant until puberty.
If male, the Wolffian ducts grow into the vas deferens and the seminal vesicles, the testes continue developing (in the abdomen), and the prostate gland and scrotum also develop. The testes descend into the scrotum during the 7th and 8th months of pregnancy.
Since the development of external genitalia is hormone dependent, disruption of normal endocrine (hormonal) function can negatively affect this process (read Teratogens).
Sex identification of a fetus is normally determined between 18 and 22 weeks of pregnancy via ultrasound during the second trimester anomaly scan. Although technology is improving quickly, it is not yet possible to accurately and reliably observe external genitalia in the first trimester – even with 3D ultrasound. Correct identification is also dependent on the ultrasound technician, machine, and fetal position.
Fetal sex may begin to be more accurately determined after 14 weeks of pregnancy.
Although debated, some research indicates that first trimester identification can be done from 11 weeks of pregnancy using the direction of the genital tubercle.
The genital tubercle is a tissue of the reproductive organs that is positioned differently depending on the sex of the fetus that will eventually develop. It either elongates to become the penis or grows very minimally to be the clitoris. The downward direction of the tubercle is considered a female while the upward direction is considered male.
Another commonly held belief is that the female fetus has a more rapid heart rate than the male fetus, and the early heart rate can be used to attempt to determine the sex. Numerous studies refute this belief, indicating there are no significant differences between heart rates of male or females, and certainly not enough difference as a reliable way to determine fetal sex.
Pregnant women who have any questions or concerns regarding the development or identification of their baby's reproductive organs should talk with their HCP.
Although the cell-free test may be attractive for women who want to learn their baby's sex as early as possible, it is not routine, may not be covered by some insurance companies, and is usually only offered to women whose pregnancies are considered high-risk for certain genetic conditions.