Which parent determines the sex of a baby?

It is the father’s sperm cell that determines the sex of the baby at the point of conception (when the sperm fertilizes the egg).

Females carry XX sex chromosomes, with each egg containing an X chromosome. Males carry XY chromosomes, with each sperm cell containing either an X or Y sex chromosome.

If a sperm with an X sex chromosome fertilizes the egg, the baby will be female (XX). If a sperm with a Y sex chromosome fertlizes the egg, the baby will be male (XY).

Here, we take a closer look at how a baby’s sex is determined.

How is a baby’s gender determined?

An individual’s DNA is made up of 23 pairs of chromosomes (46 chromosomes in total) with one copy of each chromosome inherited from each biological parent – including one pair of sex chromosomes: females carry XX sex chromosomes, whereas males carry XY sex chromosomes.

Egg and sperm cells are reproductive cells, known as gametes, that each contain half of an individual’s DNA (23 chromosomes in total). Babies are created when a sperm cell (containing 50% of the biological father’s DNA) fertilizes an egg (containing 50% of the biological mother’s DNA) to create an embryo with a full complement of DNA.

A baby’s biological gender is determined by the sex chromosomes they inherit. As females carry XX sex chromosomes, all of their gametes (egg cells) will contain one X sex chromosome. As males carry XY sex chromosomes, 50% of their gametes (sperm cells) will contain an X sex chromosome and the other 50% will contain a Y sex chromosome. Fertilization occurs when a male and female gamete join together.

If a sperm with an X sex chromosome fertilizes the egg cell containing an X chromosome, the baby will be female (XX). If a sperm with an Y sex chromosome fertilizes the egg cell containing an X chromosome, the baby will be male (XY).

Which parent determines the sex of a baby?

As females carry XX sex chromosomes, all of their egg cells will contain an X sex chromosome. Because males carry XY sex chromosomes, 50% of their sperm cells will contain an X sex chromosome and the other 50% will contain a Y sex chromosome.

If a sperm with an X sex chromosome fertilizes the egg cell containing an X chromosome, the baby will be female (XX).

If a sperm with an Y sex chromosome fertilizes the egg cell containing an X chromosome, the baby will be male (XY) and therefore it is the father’s sperm cell that determines the gender of the baby.

Are babies born or start out as one sex in the womb?

The gender of a baby is determined by the egg and sperm cells (and the sex chromosomes that they carry) that join together to create an embryo – so a baby’s gender is determined from conception.

All babies start to develop in the same way and it is difficult to visibly distinguish between male and female fetuses before week 6. As the baby grows though, male- or female-specific traits begin to develop to allow differentiation.

How can you find out the sex of a baby?

If you don’t want to wait until the birth to find out the gender of your baby, there are some options available that can determine the sex of a baby during pregnancy.

An early baby gender DNA test can be carried out from as early as 6 weeks into a pregnancy. It is 100% safe and risk-free for both mother and baby, and will produce highly accurate results. Using a non-invasive blood sample from the expectant mother, specialist tests analyze cell-free DNA (cfDNA) from the baby that is circulating within the mother’s bloodstream. Examination of this DNA can determine whether the baby is male or female by the presence or absence of the Y chromosome.

A baby’s gender can also be determined when carrying out prenatal testing to detect genetic abnormalities such as Down’s syndrome; these tests can be carried out using a sample collected through chorionic villus sampling (CVS) or amniocentesis. CVS uses a tiny sample of placental tissue and amniocentesis uses a small sample of amniotic fluid to obtain genetic material from the baby for testing and diagnosis. These methods are invasive and carry the risk of miscarriage, so are often only offered to pregnant individuals who are at higher risk of having a baby with a genetic condition.

Traditionally, ultrasound scans are used from the sixteenth week of pregnancy to determine the gender of a baby. Ultrasound sonographers carry out a visual check of the baby to check their well-being and can look at the baby’s genitals to determine whether the baby is male or female. As ultrasounds are a visual check rather than a genetic test, there is always the risk that sonographers might incorrectly predict the baby’s gender.

Generally, the later in pregnancy an ultrasound is carried out, the more likely the results are to be accurate. Ultrasound scans are particularly beneficial when carrying non-identical twins or multiples; genetic tests screen for the presence of the male-specific Y chromosome to indicate the baby is male so these tests can determine that at least one of the babies is male or that all babies are female but cannot determine the sex of each baby individually.