What Determines the Sex of a Baby? The Science of X and Y Chromosomes

What determines the sex of a baby is the chromosome in the sperm: an egg always carries an X, so a sperm carrying an X typically makes a girl (XX) and a sperm carrying a Y typically makes a boy (XY), decided at the moment of fertilization.

It is one of the first questions people ask after a positive test, and the science is simpler than most of the myths. Sex is set by a single chromosome at conception, and a single gene on the Y chromosome, called SRY, steers development weeks later.

Below: the X and Y basics, who “decides”, how sex organs develop, the real odds, why timing tricks fail, and when tests can tell you.

What Determines the Sex of a Baby? The Short Answer

That is the whole mechanism in one paragraph, and it surprises many parents. Nothing the pregnant parent eats, does or feels changes it, and the sex does not shift later in pregnancy. StatPearls, a clinical reference hosted by the National Library of Medicine, explains that the sex of the embryo is set by the combined genetic material from the sperm and egg at fertilization.

Chromosomes 101: X, Y and the 23 Pairs

MedlinePlus Genetics, from the National Library of Medicine, explains that the sex chromosomes form one of the 23 pairs of human chromosomes in each cell.

• The X chromosome: a large chromosome that everyone typically carries at least one copy of.
• The Y chromosome: much smaller. NHGRI notes the Y is about one third as long as the X and has around 100 protein coding genes, the fewest of any chromosome.
• The typical patterns: MedlinePlus Genetics describes people with two X chromosomes (46,XX) as typically developing female characteristics and people with one X and one Y (46,XY) as typically developing male characteristics.

Each parent passes on one chromosome from every pair. NHGRI puts it simply: one set of chromosomes comes from the mother and the other from the father. Since the egg’s sex chromosome is always an X, the variable piece is the sperm.

Does the Mother or the Father Determine the Baby’s Sex?

Biologically, the sperm does. NHGRI states that in fertilization, sperm can contribute either an X or a Y chromosome, while eggs always contribute an X. That means:

• Egg (X) plus sperm carrying X: XX, typically a girl.
• Egg (X) plus sperm carrying Y: XY, typically a boy.

This is worth saying gently, because women have sometimes been blamed for “failing” to have a son. No parent chooses which sperm reaches the egg. It is chance at the moment of conception.

Fertilization happens around ovulation, before most people know they are pregnant. Pregnancy weeks are counted from the first day of your last period, so conception falls around the time your calendar calls 2 weeks pregnant. By your positive test, the chromosomal sex is already set.

The SRY Gene: How Sex Development Unfolds

The real switch is one gene on the Y chromosome called SRY (sex determining region Y). MedlinePlus Genetics explains that SRY makes a protein that acts as a transcription factor: it binds to DNA and helps control the activity of other genes. NHGRI describes it as turning on a set of other genes that lead the embryo to develop certain sex characteristics, such as testes.

According to StatPearls, the early gonads are bipotential, meaning they can become either testes or ovaries, and fetal gonads do not take on male or female characteristics until about week 7 of gestation. When SRY is present and working, the gonads become testes. In its absence, they become ovaries. MedlinePlus Genetics adds that the SRY protein also helps suppress development of the uterus and fallopian tubes.

So do all babies start out female? Not exactly: StatPearls describes the early gonads as bipotential, not female. If you want to follow along week by week, see 10 weeks pregnant and 12 weeks pregnant, when many of these changes are underway.

Boy or Girl Odds: Is It Really 50/50?

Close, but not quite. Slightly more boys than girls are born. A National Center for Health Statistics report from the CDC found that the U.S. sex ratio at birth ranged from a low of 1,046 male births per 1,000 female births (in 1991 and again in 2001) to a high of 1,059 per 1,000 (in 1946). In everyday terms, that is roughly 105 boys for every 100 girls.

For any one pregnancy, that works out to a little over a 50 percent chance of a boy and a little under 50 percent for a girl. Having two or three children of one sex does not make the next baby “due” to be the other: each conception is its own event.

You may also hear that boys or girls “run in families.” Researchers are still studying whether some families lean slightly one way, and the findings are not settled. What is settled for your purposes: no family pattern, number of siblings or past birth can tell you the sex of your next baby. Only testing can.

Can You Influence the Sex of Your Baby? Timing, Shettles and Other Myths

The most famous sex selection plan is the Shettles method, which claims that sperm carrying a Y chromosome swim faster but die sooner, so having sex close to ovulation favors a boy and having sex a few days earlier favors a girl.

The best known test of the timing idea comes from a 1995 study in the New England Journal of Medicine by Allen Wilcox and colleagues at the National Institute of Environmental Health Sciences. They followed 221 healthy women trying to conceive and pinpointed ovulation with daily urine samples across 625 menstrual cycles. Their conclusion on sex was plain: cycles producing male and female babies had similar patterns of intercourse in relation to ovulation. In other words, timing did not predict whether the baby was a boy or a girl.

• Timing sex near ovulation for a boy: not supported by the Wilcox study, which found similar timing for boys and girls.
• Diets (salty foods for a boy, dairy for a girl): we could not find reliable evidence that any diet changes the sex of a baby. It is set by the sperm at fertilization.
• Sex positions, orgasm or pH tricks: we found no reliable evidence that any of them change the odds.
• Gender prediction calendars: fun, but they cannot change biology.

If a sex-linked genetic condition runs in your family, talk with your doctor or a genetic counselor before trying to conceive.

Differences of Sex Development

For a small number of babies, chromosomes, gonads and anatomy do not line up in the typical XX or XY pattern. These are called differences of sex development (DSDs), and you may also hear the word intersex. MedlinePlus notes that when an ultrasound shows genitals that are not clearly male or female, cfDNA screening may be used to learn more.

MedlinePlus Genetics describes several examples linked to the SRY gene itself:

• Swyer syndrome: a person has a 46,XY karyotype but female-typical characteristics. SRY variants are found in about 15 percent of cases; without a working SRY protein, testes do not form, but a uterus and fallopian tubes do.
• 46,XX testicular DSD: a person has a 46,XX karyotype but develops male characteristics. Most cases happen when SRY is moved from the Y chromosome, usually onto an X, during sperm formation in the father.
• Ovotesticular DSD: a person has both ovarian and testicular tissue. SRY translocations cause about 10 percent of cases.

NHGRI also notes that some people have extra sex chromosomes, such as XXY, XYY or XXX. These examples show why doctors say XX and XY are what typically lead to female and male development. If a scan or blood test suggests something unexpected, your care team or a genetic counselor can explain it.

When and How You Can Find Out Your Baby’s Sex

There are two main ways your baby’s sex shows up during pregnancy:

Cell-free DNA screening. MedlinePlus explains that cfDNA screening can be done as early as the 10th week of pregnancy. Ask your provider whether the lab will report sex and whether you can choose not to hear it.

The anatomy scan. Cleveland Clinic describes the 20-week ultrasound, often called the anatomy scan, as taking place between 18 and 22 weeks. Seeing the sex depends on position. If you want a surprise, Cleveland Clinic suggests telling the technician before the scan starts. Our 20 weeks pregnant guide covers what else the scan looks at.

What about earlier scans or home kits? Early in pregnancy the external genitals are still forming (StatPearls notes the scrotum is formed by about week 12), so a very early ultrasound guess can be hard to read. If you are tempted by a home kit, ask your provider first; testing ordered through your care team comes with someone to interpret the result.

Old Wives’ Tales, Gender Reveals and Keeping It Fun

Bump shape, cravings and the ring test cannot tell the sex, but they make a fun game while you wait. Our roundup of old wives’ tales for boy or girl explains where each one came from.

Once you know (or decide to stay surprised), you might want to celebrate. Browse gender reveal ideas and gender reveal games, and start a short list from baby girl names or baby boy names. If you are still deciding when to share the bigger news, read when to tell people you are pregnant.

Related Betteroo Guides

• Old Wives’ Tales About Baby Gender
• Gender Reveal Ideas
• When to Tell People You’re Pregnant
• 20 Weeks Pregnant
• 12 Weeks Pregnant
• Baby Registry Checklist

When to Call Your Doctor or Midwife

Learning your baby’s sex is usually just exciting news. Contact your doctor, midwife or a genetic counselor when:

• You have a family history of a sex-linked genetic condition: ideally before or early in pregnancy, so you can discuss testing options.
• A cfDNA screening result comes back unexpected or “high risk”: MedlinePlus stresses that it is a screening test; a positive result means a higher chance, not a diagnosis, and diagnostic testing such as CVS or amniocentesis is needed to confirm.
• The cfDNA result and the ultrasound do not match: for example the blood test reports a Y chromosome but the scan looks female, or the reverse. Your provider can explain possible reasons and next steps.
• The anatomy scan cannot clearly show the genitals or flags any concern: Cleveland Clinic notes your provider may order more tests if something on the scan needs a closer look.
• You feel anxious or disappointed about the result: tell your provider, who can connect you with support.

For heavy bleeding, severe abdominal pain or a change in your baby’s movements, call your provider right away; call 911 in an emergency. This article is general education, not medical advice.

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