Which parent determines the sex of a baby?

Casey Randall Alphabiolabs

By Casey Randall, Head of Genetics at AlphaBiolabs
Last reviewed: 03/01/2024

A baby’s gender is determined by their sex chromosomes. Typically, females carry XX sex chromosomes whereas males carry XY sex chromosomes. Parents can choose to wait until birth to find out the sex of their baby or may want to find out during their pregnancy to help them plan and prepare.

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) fertilises 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. Fertilisation occurs when a male and female gamete join together.

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

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

What sex do babies start as?

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 foetuses 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 8 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 analyse 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.

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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.

Alternatively, a non-invasive prenatal genetic test (NIPT) may be used to screen for genetic abnormalities. NIPT screening can be carried out from the tenth week of pregnancy (confirmed by an ultrasound scan) and uses only a blood sample collected from the mother instead of a CVS or amniocentesis sample to identify pregnancies that are high-risk for genetic abnormalities.

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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.

Are babies always XX or XY?

In most cases, females carry XX sex chromosomes and males carry XY sex chromosomes but there are some conditions that affect the sex chromosomes, where the baby inherits something other than the typically observed XX or XY – these conditions are confirmed by carrying out genetic testing.

An Advanced AlphaBiolabs IONA test can be carried out using a non-invasive blood sample from a pregnant individual and the test is used to screen all chromosomes (including the sex chromosomes) to identify any extra or missing copies of a chromosome which indicate a genetic abnormality.  For example, Down’s syndrome is caused by an extra copy of Chromosome 21.

The most commonly observed sex chromosome disorders (known as aneuploidies) are:

  • Turner’s syndrome (Monosomy X) where a female child only receives one copy of the X chromosome – X instead of XX
  • Triple X syndrome (Trisomy X) where a female child inherits three X chromosomes – XXX instead of XX
  • Klinefelter’s syndrome (XXY) where a male child is born with an extra copy of the X chromosome.
  • Jacob’s syndrome (XYY) where a male child is born with an extra copy of the Y chromosome.

Where can you find out the baby’s sex?

An early baby gender DNA test or non-invasive prenatal genetic test (NIPT) requires a blood sample to be collected from the pregnant individual and returned to the laboratory for testing.

AlphaBiolabs have an extensive network of sample collectors across the country and can arrange to collect the blood sample at your home, at an AlphaBiolabs walk in centre, or at another convenient location.

You will receive a confidential email with a report containing your results within 3 days of your samples being received at our laboratory, or you can even upgrade for same-day results!

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Casey Randall, AlphaBiolabs

Casey Randall

Head of Genetics at AlphaBiolabs

Casey joined the AlphaBiolabs team in 2012 and heads up both the DNA and Health testing teams.

An expert in DNA analysis and a member of the International Society for Forensic Genetics (ISFG), Casey holds an MSc with Distinction in DNA Profiling and a First-Class BSc with Honours in Forensic Science.

Casey is responsible for maintaining the highest quality testing standards, as well as looking for ways to further enhance the service that AlphaBiolabs provides and exploring new and innovative techniques in DNA analysis.

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