What Influences a Baby's Sex?

Some suggest conception timing and stress levels affect the sex, but it really comes down to biology.

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Before labor and delivery, and even before pregnancy, people might want to know what determines the sex of a baby.

A baby's biological sex is often used interchangeably with their gender, but the two are not the same. Sex refers to a person being assigned male, female, or another sex biologically at birth. Gender refers to sociocultural roles or activities someone may be expected to have or do depending on their sex.

Situations like conception timing and stress levels are suggested to play a role in what sex a baby will be. However, that outcome really depends on biological processes. Read on to learn more.

Editor's note: Health recognizes that not everyone who is female was born with female reproductive organs and that not everyone who is male was born with male reproductive organs. Health also recognizes that people may not identify as any one sex or gender. The information in this article is based on how researchers present their results, and the gender- and sex-based language used most accurately reflects their research design and outcomes.

When Does Biological Sex Develop in the Womb?

It takes a few weeks for a baby's biological sex to develop during pregnancy. The beginning of cell growth to determine male or female biological features happens about five weeks after an egg has been fertilized.

By week 14, biological sex features are fully developed. It's not until later that future parents can know the sex of the baby. They can find out during the mid-trimester ultrasound—also called a second-trimester anatomy scan or anomaly scan—performed between 18 and 22 weeks.

What Determines Sex?

A person's sex will depend on the presence of sex chromosomes, which are part of DNA.

There are two human sex chromosomes: the Y chromosome and the X chromosome. Usually, individuals have one sex-chromosome pair in their cells. People assigned male at birth will have an X and a Y chromosome; those assigned female at birth will have two X chromosomes.

There can be sex chromosome changes related to how the chromosomes are structured or the number of Y chromosomes available like:

  • 46, XX testicular difference of sex development
  • 47, XYY syndrome
  • 48, XXYY syndrome
  • Y chromosome infertility

These changes can affect a person's fertility, resulting in developmental or behavioral disorders and other health problems.

Suggested Factors in Determining a Baby's Biological Sex

The odds of delivering a boy versus a girl are simply the luck of the draw. Globally, male births only slightly trump female births—about 107 boys for every 100 girls. In the United States, the ratio is around 105 boys to 100 girls.

Research has suggested some possible indicators that could influence the sex of a developing fetus. Just remember the ideas below may only make a small difference, if any at all.

In-Vitro Fertilization

Babies conceived via assisted-reproduction techniques may be more likely to be a boy. One study found that more male infants were born via frozen-thawed embryo transfers using blastocysts—the beginning stages of embryos.

Of note, one researcher provided more insight regarding the study. The researcher said it was important to understand that more boys had been born in general without using in-vitro fertilization (IVF). This meant that IVF outcomes actually resulted in fewer birthed boys.

Family History

People often try to guess the sex of an unborn baby based on the number of boys and girls already in the family, or the number of brothers or sisters each parent has. Yet, one Swedish study determined that genetics did not contribute to the prevalence of one sex over the other in families.

Timing of Sexual Intercourse

Individuals have suggested having sex at certain times during the menstrual cycle could sway the outcome of a baby's sex.

The idea is that a person could end up with a baby born male or female based on how closely they had sex around the time of ovulation. Though research is limited on the connection, this method has been found to have undependable results.

Stress Levels

Inflammation is a consequence of stress and can affect the health of a pregnancy. Stress levels have also seemingly played a role in whether a baby's biological sex is male or female.

Research has found that more girls than boys have been born when mothers have been under stressful conditions. One study said that male babies experience more of an impact from pregnancy inflammation and may more likely give way to that impact.

Diet and Nutrition

Some research found links between maternal nutrition and the biological sex of a child, like eating cereal leading to the birth of boys. Also, one review found:

  • No nutrition effect on sex ratio except for malnutrition, when fewer boys were born
  • High sodium and potassium intake linked to male babies
  • High magnesium and calcium intake linked to female babies

Still, the researchers concluded that there wasn't enough research available to confirm the nutrition-and-sex-determination link.

Pregnant people should engage in healthy eating patterns. This includes ensuring certain foods are in their diet, like fruits, vegetables, and whole grains. They should also eat enough calories for their individual case, though more daily calories may be required as pregnancy progresses.

The Role of Gender Identity Versus Biological Sex

Gender identity refers to how people see themselves in terms of their biological sex. They may consider themselves as male, female, both sexes, or neither one of them. A person's identity may also influence what they prefer to call themselves or what others call them.

Because different gender identities exist, a person can have an identity that may or may not align with expectations for their biological sex. Ways to be supportive of individuals with different gender identities include:

  • Being aware of any gender identity attitudes you may have
  • Communicating with them
  • Educating yourself about gender identities
  • Not making assumptions about their identity
  • Seeking and getting support for your reactions and feelings
  • Using their chosen names and pronouns—and asking if you're unsure

A Quick Review

Some factors, like family history and nutrition, have been noted as possible indicators for biological sex determination. However, these factors have little influence on what a child's sex will be; instead, sex chromosomes are the best determinants.

Also, gender is different from biological sex. Future parents should remember that while humans are born with male or female genitalia, it does not mean that is how the child will identify themselves.

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Health.com uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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