When Does A Baby's Sex Develop in the Womb? Experts Explain

Discovering that you are pregnant can feel like a whirlwind, to put it mildly. Suddenly, your body starts feeling different, and your mind is full of thoughts and questions: Are you mentally prepared for all of this? Should you start thinking about names or nursery ideas yet? You might also be curious about your baby’s sex and when you’ll be able to find out.

Charles Carlsen, an OB-GYN and diagnostic medical sonographer, explains that these feelings are completely normal—many parents are eager to learn whether they’re having a boy or a girl. “Parents want to feel close to their baby from the start,” Carlsen says. “Finding out the sex helps them prepare in many ways, like choosing names, shopping for baby clothes or planning the nursery. It’s a way for parents to begin bonding with their baby even before birth.”

If you’re wondering when a baby’s sex develops in the womb, our experts explain the timeline of fetal development—how and when a baby’s sex can be detected, the best methods to determine it and what factors might affect when parents find out.

When and how a baby's sex develops in the womb

Here are the basics. Keep reading for detailed explanations of each stage.

• Sex is determined at conception: The father's sperm contributes either an X or Y chromosome, establishing the biological sex—XX for female, XY for male.
• Sexual differentiation begins around week 6: Physical development of reproductive organs starts approximately six weeks after fertilization.
• Early development is undifferentiated: For the first weeks, embryos follow a “default” female path unless male hormones are triggered by the SRY gene on the Y chromosome.
• Hormones shape development: The presence of testosterone and AMH directs male development; their absence leads to female development.
• Ultrasound can reveal sex around 14–18 weeks: Genital differences become visible via imaging, though clarity varies.
• Genetic testing offers earlier accuracy: Tests like SneakPeek or Prequel can detect Y chromosome material in maternal blood as early as 6–8 weeks.
• Sex is not the same as gender: Experts stress the difference between biological sex (chromosomes, hormones, anatomy) and gender identity (social roles, self-perception).
• Focus on fetal health over sex: While exciting, knowing the sex should not overshadow the importance of prenatal care, developmental milestones and emotional bonding.

How biological sex is determined

To understand how a baby's sex develops in the womb, it helps to start with the basics of genetics and the sex determination system.

Endocrinologist Lisa Becht explains that in many species, including humans, chromosomes determine sex.  Humans have 23 pairs of chromosomes (46 in total), with one chromosome in each pair inherited from the mother’s egg and the other from the father’s sperm. "The 23rd pair are sex chromosomes: X and Y," Dr. Becht explains. "Typically, females have XX, and males have XY."

Other factors, though, can influence your baby's sex. "Genetic abnormalities, such as Trisomy 21—that's three copies of chromosome 21 instead of two—or Turner Syndrome—only one X chromosome—are examples of conditions involving abnormal chromosome numbers," Becht says.

Biological sex vs. gender identity

Carlsen also says that it's important to recognize that sex is different from gender. "Biological sex refers to the physical characteristics and chromosomes determined at conception," he explains. "Gender is the social role and identity within society—a more fluid and self-determined aspect of identity. Parents would do well to notice this distinction, as it helps in understanding how their child develops their sense of self over time."

Dr. Dallas Reed, an OBGYN and leading medical geneticist at Tufts Medical Center in Massachusetts, further emphasizes that sex chromosomes do not determine someone's gender. "Biological sex is based on chromosomes, hormones and anatomy—male, female, intersex," she explains. "Gender is how people identify and how society labels roles, behaviours and expectations—for example, woman, man, nonbinary."

As Carlsen summarizes it, determining a child's biological sex is a strong starting point for tracking their development, but it's just that: a starting point. "While biological sex is an early indicator, the development of a child is far more than this," he says. "It also includes things like personality, temperament and intellectual development, all of which add to the sum of who they are. Accepting the whole spectrum of the development of a child facilitates a broader base for their development."

The moment of conception

Sex, Becht explains, is determined by chromosomes, which are the strands of DNA that carry genetic information. "A female egg always carries an X chromosome, while sperm can carry either an X or a Y chromosome," she says. "If the sperm contributes an X, the baby is female [XX]. If the sperm contributes a Y, the baby is male [XY]."

But when exactly does a baby's sex develop? "A human infant's biological sex is determined at the moment of conception, when chromosomes from the parents are mapped out—XX in girls and XY in boys," Carlsen says. "This genetic template is determined at fertilization, but physical development of sexual characteristics takes a few weeks longer."

The developmental process where a single, undifferentiated embryo develops into a male or female individual is known as sexual differentiation.

Early embryonic development

Shalini Shah, a doula, perinatal educator and lactation consultant, explains that for the first several weeks after conception, there are no clear physical signs of sex. This period is known as the indifferent stage, and is crucial for understanding how sex is determined.

"The internal organs haven’t differentiated, and the external genitals all look the same," Shah summarizes. "Everyone has what’s often described as a 'female template' by default—meaning that without a strong hormonal signal to change course, the body will develop along a female path. Even if an embryo has XY chromosomes, you wouldn’t be able to tell just by looking. That’s why early ultrasounds can’t determine sex, and why genetic testing is the only reliable way to know at this stage."

During this stage, embryos have what are called 'indifferent' gonads (gonads are the reproductive glands responsible for producing sex hormones). The gonadal tissue hasn't yet become ovaries or testes, and the Wolffian and Müllerian ducts have yet to take shape. "The Wolffian ducts can turn into parts of the male reproductive system, and the Müllerian ducts can become parts of the female system," Shah explains.

What happens next depends on hormones. According to Shah, "If the embryo starts producing testosterone and anti-Müllerian hormone (AMH), the Wolffian ducts are supported and the Müllerian ones shrink away. If those hormones aren’t present, the opposite happens. The indifferent gonads become ovaries, and the Müllerian ducts [and] continue developing. It’s all about which signals arrive and when."

The role of genes and hormones in differentiation

Genes and hormones have a big effect on how sex develops. Shah explains that a gene called 'sex-determining region Y protein' (or SRY for short) acts as the main trigger for whether a baby develops female or male reproductive parts. "If the SRY gene is present, which usually means the baby has XY chromosomes, it starts a process that leads to male-producing development. If the gene isn’t there, which is the most common for most babies with two X chromosomes, the body follows the natural 'default' path and develops female reproductive parts," she explains.

"Once the testes form, they start making hormones, especially testosterone, which help shape the baby’s male anatomy," Shah goes on. "These hormones also stop female reproductive structures from developing, keeping development moving in a male direction." Once the testes are in place, they start producing testosterone and another hormone called anti-Müllerian hormone, or AMH. "Testosterone helps grow the male internal and external reproductive organs like the penis and prostate," Shah says. "AMH also causes the tissues that would have formed female reproductive parts to shrink away, ensuring male development continues."

Without the SRY gene, Shah says, the baby’s gonads will instead develop into ovaries. "The body then follows the 'default' female development route: Forming ovaries, a uterus, fallopian tubes and external female genitalia like the vulva," she explains.

When biological sex becomes physically apparent

Becht explains that up until seven weeks of pregnancy, all fetuses have a structure known as the genital ridge. After this point, their sex organs begin to develop differently. The genital ridge transforms into the penis in males because of testosterone, while in females, it becomes the clitoris," she explains. "For the first 14 weeks, the clitoris and penis are about the same size. After that, the penis keeps growing longer."

Around 14 weeks, Becht notes that it is usually possible to tell the baby's sex using an ultrasound. She explains, "The changes in the genital ridge mean you can usually see the differences between boys and girls by this stage," she says, going on to caution: "However, it's not always certain, especially if the ultrasound images are not clear. That's why doctors often prefer to wait until between 16 to 18 weeks to determine the baby's sex with more confidence."

Dr. Carlsen says sexual differentiation begins around six weeks after fertilization.

The bottom line

While understanding sex is important, Reed emphasizes the importance of focusing on the baby's overall health, rather than their biological sex, during pregnancy. “While it can be exciting to find out what you’re having in order to start bonding with the baby, this can sometimes make people put too much emphasis on gender roles or stereotypes," she says. "The real purpose of prenatal care is to support the safe and healthy development of the baby, not to make assumptions or set expectations based on its sex. Knowing the baby’s sex does not give us information about its personality, development, or health.”

To do this, Carlsen encourages parents to pay attention to the positive milestones of fetal development, such as the growth of the heart, brain and limbs. “Following the recommended prenatal tests, eating a healthy diet, staying active and creating a positive emotional environment are all important ways to help your baby grow well,” he says.

FAQs

1. Are ‘gender prediction’ methods like old wives’ tales accurate? Dr. Reed explains that most traditional methods for predicting a baby’s sex—such as assessing food cravings, severity of morning sickness, belly shape, skin or hair changes and other such methods—are not scientifically accurate. T

2. What is the best way to determine the sex of your baby?

The most reliable methods for determining fetal sex are genetic screening tests, such as the SneakPeek test (available as early as 6 weeks) and the Prequel screening test (available as early as 8 weeks). They're so effective, Reed says, because they detect fetal DNA in the mother’s blood. "If Y chromosome material is found, the test predicts a male fetus," she explains.

3. When does your baby’s brain and nervous system begin to develop?

Fetal brain and nervous system development actually begins before most people even realize they are pregnant, Reed says. "This is why prenatal vitamins containing folic acid are recommended even before conception, to ensure adequate nutrient levels during the critical early weeks of pregnancy," she says.

Experts

1. Charles Carlsen is an OB-GYN and Registered Diagnostic Medical Sonographer.
2. Dr. Dallas Reed is an OBGYN and medical geneticist who serves as the Chief of Genetics at Tufts Medical Center and as an Associate Professor at Tufts University School of Medicine.
3. Shalini Shah is a full-spectrum doula, perinatal educator, and board-certified lactation consultant (IBCLC).
4. Dr. Lisa Becht, MD, FACOG, is a board-certified reproductive endocrinologist.