What determines the sex of the baby?

by | April 2022

Some expecting parents love the mystery of waiting until the delivery to discover their baby’s sex. Others, not so much. Your baby’s genetic information — including their sex — is determined at conception, but many parents don’t find out if they’re having a boy or girl until the second trimester (or later). Here’s a primer on what determines your baby’s sex and how that influences fetal development.

What’s the difference between sex and gender?

First of all, we should clarify that a baby’s genetic sex may differ from the gender they identify with as they grow up. Sex is a label assigned at birth, and it’s usually based on a baby’s external anatomy. 

Gender, on the other hand, is not something that can always be concretely pinpointed by tests or examinations, especially given all of the diversity in human chromosomes, genes, hormones, internal and external anatomy, and other experiences!   Gender is a complex concept that involves how someone sees themself and how they want the world to see them. 

What determines if your baby is a boy or a girl?

A baby’s sex is determined by the chromosomes they inherit from their genetic parents. These chromosomes are passed down through the combination of an egg cell with a sperm cell. Although egg cells and sperm cells contain many chromosomes, the “sex chromosomes” (X and Y) are the ones that are the most important for the process of sex organ development. 

Egg cells can only contribute an “X” sex chromosome, not a “Y” sex chromosome. Typically, millions of sperm are released during ejaculation, and it’s usually 50/50 whether a sperm carries a single “X” chromosome or a “Y” chromosome. When an  “X” sperm fertilizes an “X” egg, that conception will have “XX” sex chromosomes, which is consistent with female sex chromosomes. In contrast, when a “Y” sperm fertilizes an “X” egg, that conception will have “XY” sex chromosomes, which is consistent with male sex chromosomes. 

 Though sex chromosomes are determined at the moment of conception,  sex chromosomes don’t start to affect fetal development until about seven weeks gestation. Until this point, the pregnancy develops the same regardless of whether the sex chromosomes are XX or XY.

As the pregnancy grows, gene instructions on the sex chromosomes trigger the development of the internal and external reproductive system, which is influenced by the release of specific hormones. By 18 to 20 weeks gestation, a baby’s genitals are typically developed enough for an ultrasound technician to see a protrusion between the baby’s legs or not – can you guess what that means? . However, factors including the baby’s position and maternal BMI may make it difficult to see the baby during an ultrasound.

At-home blood tests, such as the Juno Early Gender Test, can help you learn the sex of your baby as soon as seven weeks after your last menstrual period. These tests look for fetal DNA in your blood and analyze it to see whether your baby’s sex chromosomes are consistent with XX (female) or XY (male).

Can you choose the sex of your baby?

There’s no shortage of lore about timing intercourse and trying different sex positions to influence the sex of your baby. However, there’s no conclusive evidence that these methods affect the likelihood of an X-chromosome sperm or Y-chromosome sperm winning the race to fertilize the egg. 

There’s no proven way to choose your baby’s sex when conceiving the old-fashioned way, but looking at the family tree on the sperm side might help predict whether you’re having a boy or girl. According to a 2008 study that looked at hundreds of years of family trees, a male with many brothers and no sisters may be more likely to have a higher percentage of Y-chromosome sperm, increasing their chances of having a boy. The opposite is true of males that have mostly sisters.

Note: If you’re trying to conceive through in vitro fertilization (IVF) and are planning to use a technology called preimplantation genetic testing (PGT), you may be offered the option of knowing which embryos have XX chromosomes vs.  which have XY chromosomes. Different clinics, states, and countries have varying policies (and sometimes even laws) regarding whether the practice of “sex selection” – that is, choosing which embryo to transfer based on the sex from PGT – is allowed.

You may not be able to choose the sex of your baby, but you don’t always have to wait until your second trimester to find out if you’re having a boy or girl.