How is Sex Determined in a Fetus?

Having a baby is one of the most miraculous things we can do as human beings, but we are also naturally curious creatures. Waiting 9 months to find out whether we’re having a boy or a girl can be nerve-wracking, and also makes planning the color scheme of the nursery much more difficult. Fortunately, there are a few ways to determine whether you’re having a boy or a girl.

Throughout history, there have been many “methods”, most of which are medically questionable or downright false, to determine the gender of your future child, but our modern technological advancements have made this gender issue much more clear cut. So…. how is sex determined?

Short answer? Blood tests or ultrasounds, but there is much more to the story.

Determining Gender in the Past: Fact and Fiction

For hundreds of thousands of years, humans beings came up with different ways to determine what the sex of their baby would be. Ranging from paying attention to what foot you walked with first to the weight gain of the father, people truly believed in these methods. For the record, if you stepped first with your right foot, it would be a boy, and if you stepped first with your left, you were expected to have a daughter. Also, if the father gains weight during the pregnancy, it is more likely to be a girl. These approaches were not based on science, but tradition and belief is strong. The question is… are they strong enough to affect a fetus’ gender?

The placement of the baby was also believed to be a trustable factor; a higher baby bump was going to be a girl, while a lower bump signified a boy. Cravings for food were also believed to be indicative; sweet cravings meant a girl, while savory cravings signaled a son on the way.


Some of these old beliefs do have some validity, once science caught up with fiction. For example, many people believed that morning sickness in the first three months meant you were having a daughter, while pain in the second trimester was a sign of a son. As it turns out, modern research has shown that long-term morning sickness (throughout your pregnancy), is a reliable indicator of a daughter.

However, despite the wide range of these beliefs, the fact of the matter is that reliably determining the sex of a fetus goes far beyond superstition.

Chromosomes and Conception

As most of you know, sex is dependent on which chromosomes you carry as a fetus. If an egg is fertilized by sperm with an X chromosome (female chromosome), then it will be a girl, whereas if the egg is fertilized by sperm with a Y chromosome, then it will be a boy. Studies have shown that sperm with the X chromosome tend to move slower, due to more mitochondria being present, but have longer lifespans than Y-chromosome sperm, which have higher motility rates. More mitochondria means more energy-producing capacity, which explains the longer survival rate of those sperm after insemination.

Therefore, if a woman knows when she is ovulating, and calculates when she likely conceived the child, then there is some reliability in predicting which sex the fetus will be. Sperm can move into the Fallopian tubes in as little as 20 minutes, and can survive for 4-7 days inside of a woman’s reproductive organs. However, there is still a great deal of variability in this manner of “prediction”. The best way to learn the gender of your baby is to turn to the experts…

Ultrasound Imaging and Blood Tests

In the first few weeks of fetal development, the future child is much too small for an ultrasound to help very much, but going in for an ultrasound with a trained sonologist between 16-20 weeks is a reliable way to determine the sex of the fetus. By imaging the fetus, a doctor should be able to see a penis or labia, which would denote a boy or girl, respectively.

Seeing a labia in a sonogram is more definitive than not seeing a penis, as every fetus develops slightly differently. The visibility of these sex organs is also dependent on the thickness of the abdominal wall and the fetal position, so in certain cases, sex can’t be determined until nearly the 7-month mark!

On the other hand, rapidly developing fetuses can sometimes be identified as early as 11 or 12 weeks, which is good news for those parents who simply can’t wait to know what sex their little bundle of joy will be. Ultrasound accuracy in determining sex is between 95-100% when a good image is available.

Some people are even more eager to know their child’s gender, so waiting until 3 or 4 months simply isn’t an option. In recent years, primarily in Europe, a blood test called non-invasive prenatal testing (NIPT) has been employed to determine chromosome condition and sex of infants. These highly reliable blood tests can identify the chromosomal arrangement of the fetus and give a determination of fetal sex, ranging from 95-99% accuracy for boys and girls, respectively.

These types of blood tests replaced earlier, more invasive tests, such as amniocentesis or chorionic villus sampling. These tests could determine gender by 10 or 11 weeks, but slightly increased the chances of miscarriage. Most people who undergo these types of blood tests do so in order to check for genetic disorders, such as Down syndrome, that are often linked to the X chromosome.

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Parents with family histories of genetic disorders commonly rely on these blood tests, but they are not widely used, especially in the United States. Generally speaking, determining the sex of your fetus (if you want to know!), can happen as early as 7 weeks with NIPT, or as late as 28 weeks with a traditional ultrasound image.

That being said, if doctors intimidate you, just pay attention to what foot you step with first… after all, you’ve got a 50% chance of predicting the sex correctly!

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About the Author

John Staughton is a traveling writer, editor, publisher and photographer who earned his English and Integrative Biology degrees from the University of Illinois. He is the co-founder of a literary journal, Sheriff Nottingham, and the Content Director for Stain’d Arts, an arts nonprofit based in Denver. On a perpetual journey towards the idea of home, he uses words to educate, inspire, uplift and evolve.

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