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Boy or Girl? Early Signs of Your Baby’s Sex

We’ve all heard about the early signs that can help pregnant parents predict their baby’s sex. But do you truly know how biological sex is determined and which of these signs are reliable? Read this article to learn more about predicting your baby’s sex.

Why know the sex before delivery?

When you find out that you’re pregnant, your top priority is of course to have a healthy pregnancy and a healthy baby. However, many people want to know their baby’s sex from the moment they get a positive pregnancy test

Maybe you’ve been thinking about unisex baby names, or maybe discovering your baby’s sex could help you decide on a name and prepare for their arrival. Many parents use their child’s biological sex to decide what color clothing and nursery decorations to buy. 

When does the fetus develop their biological sex?

The process through which sex is determined is called human sexual differentiation. You probably won’t find out your baby’s sex until a few months into your pregnancy, but it was set in stone at the moment of conception.

The baby’s genes determine sex. All eggs contain an X chromosome, while sperm can have an X or a Y chromosome. If the egg is fertilized by a sperm cell carrying an X chromosome, the resulting XX embryo will be female. However, if the sperm cell has a Y chromosome, the embryo will have male XY chromosomes.

At first, all embryos look the same regardless of sex. At the fifth week of your pregnancy, your baby will have a structure called a “gonadal ridge” (or “genital ridge”). Until week seven, they will develop the precursors of the sex organs. Then, over the next five weeks, your embryo starts producing hormones that stimulate the development of their sex organs.

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All sex organs come from the genital ridge. The ovaries are equivalent to the testicles, meaning they are formed from the same cells, and the clitoris and the penis are equivalents. So once those hormones kick in, the genital ridge begins differentiating into these structures.

For most fetuses with XY chromosomes, the genital ridge starts to lengthen into a penis by week 11. Tiny buds will form the prostate around week 14, and the urinary system is formed by week 16. Testicles descend into the scrotum at around weeks 26–31, and the penis continues growing during the third trimester.

For most fetuses with XX chromosomes, primary ovaries appear around week 12. Between weeks 14 and 20, they fill up with 6–7 million primitive eggs. These eggs reduce in quantity up until birth, at which point there are about 1–2 million eggs left. Two structures called the paramesonephric ducts fuse to form the uterus and vagina around week 16 of pregnancy.

Five signs that you’re having a girl

There are lots of old wives’ tales and myths about how to predict the sex of your baby. Here’s the truth about five supposed pregnancy signs for a girl:

Carrying the baby high

One of the most common myths surrounding a baby’s sex is that having a higher bump means you’re having a girl. However, this is not true.

The biggest factors that determine the way your bump looks are your physical condition, the number of pregnancies you’ve had, your abdominal muscles, and your total weight gain during pregnancy.

Mood swings

Some people think that the estrogen produced by a female fetus can affect the mother’s temperament, causing mood swings. However, anyone who’s pregnant can experience mood swings due to their own hormonal fluctuations: these mood changes aren’t related to the baby’s sex.

Morning sickness

Another popular belief is that the copious hormones produced by a female fetus can cause more severe cases of morning sickness. But severity of nausea and vomiting varies from person to person, and even the same person can have different experiences during different pregnancies.

Having acne or dull skin

Have you ever heard the saying that a baby girl steals her mother’s beauty? Some people believe that if you’re having a girl, you’ll develop oily or dull skin and acne. This is another falsehood. Pregnancy hormones can be unpredictable, and they affect each person’s skin and hair differently.

Five signs that you’re having a boy

Craving salty foods

Research has shown that anywhere between 50 and 90 percent of all people experience food cravings at some point during pregnancy. A common myth is that a female fetus causes cravings for sweets, and a male fetus causes cravings for savory and salty foods.

The truth is that a person’s cravings are more likely to be related to their nutritional requirements than to the sex of their baby. There’s also a theory that the food a person craves may partially depend on their culture.

Carrying low

Just as carrying high is supposed to mean that you’re having a girl, it’s widely believed that a lower bump means your baby is a boy. However, there’s no evidence to back up this theory, and as stated above, the shape of your bump is determined by other factors. 

Healthier hair and skin

This is the opposite of the myth that having a girl will “take away your beauty.” Many people believe that if you’re carrying a boy, your skin will look healthier, and you’ll have thick, lustrous hair.

In reality, these changes depend solely on pregnancy hormones. Some people develop skin pigmentation or acne, while others will have thicker hair and a “pregnancy glow.”

No mood swings

Does anyone sincerely believe that having a boy means that a pregnant person will avoid mood swings? Unfortunately, it’s not true. The biological sex of your baby won’t have any impact on whether you’ll have mood swings. They’re a very common occurrence.

Medical tests to learn your baby’s sex

It’s no wonder so many people believe in these myths — they have an almost 50 percent chance of being right. These are some of the tests that your health care provider can use to determine your baby’s sex:

NIPT: Non-invasive prenatal testing

NIPT is a method of testing for the likelihood of genetic abnormalities in the first trimester. This maternal blood test poses no risk to the fetus. It’s usually used to detect Down syndrome in high-risk pregnancies, but it can also detect the sex of the baby. The accuracy of the test is around 98 percent.

Ultrasound

This is the most common way to determine your baby’s sex, and most low-risk pregnancies find out the baby’s sex this way between the 18th and 22nd week of pregnancy. It’s accurate and completely safe for the fetus.

Amniocentesis

Using a sample of amniotic fluid, amniocentesis can detect genetic abnormalities and your baby’s sex. This method is invasive and is usually done in high-risk pregnancies.

Chorionic villus sampling (CVS)

CVS is an invasive test that requires a sample of placenta to diagnose Down syndrome or other chromosomal abnormalities. It can also determine the baby’s sex as early as week 10 of pregnancy.

Preimplantation Genetic Testing (PGT)

Some parents may wish to only have a baby of a certain sex. This may be due to the wish to avoid sex-linked diseases or as a matter of preference. In certain countries, it’s legal to select the sex of your baby by undergoing IVF. Embryos are produced in the laboratory and tested for sex, after which only embryos of the chosen sex are implanted.

Amniocentesis and CVS are primarily carried out when fetal abnormalities are suspected, and both tests pose certain risks. They are not recommended as ways to determine the sex of the fetus.

Final note

It’s normal to want to know your baby’s sex, but being a healthy parent with a healthy baby is the ultimate goal. Maintain healthy nutrition during pregnancy, monitor the fluids you consume during pregnancy, stay active, and when your baby arrives, you might realize their sex isn’t that important after all!

Walsh, Kate, et al. “Maternal Prenatal Stress Phenotypes Associate with Fetal Neurodevelopment and Birth Outcomes.” PNAS, National Academy of Sciences, 26 Nov. 2019, www.pnas.org/content/116/48/23996.

Mayo Clinic Staff. “Fetal Development: What Happens during the 1st Trimester?” Mayo Clinic, Mayo Foundation for Medical Education and Research, 12 July 2017, www.mayoclinic.org/healthy-lifestyle/pregnancy-week-by-week/in-depth/prenatal-care/art-20045302.

Mayo Clinic Staff. “Fetal Ultrasound.” Mayo Clinic, Mayo Foundation for Medical Education and Research, 3 Jan. 2019, www.mayoclinic.org/tests-procedures/fetal-ultrasound/about/pac-20394149.

“Genital Ridge.” Genital Ridge - an Overview | ScienceDirect Topics, www.sciencedirect.com/topics/engineering/genital-ridge.

Hill, Mark A. “BGDB Sexual Differentiation - Late Embryo.” UNSW Embryology, 2020, embryology.med.unsw.edu.au/embryology/index.php/BGDB_Sexual_Differentiation_-_Late_Embryo.

Paxton, Steve, et al. “The Leeds Histology Guide.” The Histology Guide, www.histology.leeds.ac.uk/female/FRS_ova.php.

“Female Reproductive System: Structure & Function.” Cleveland Clinic, 19 Jan. 2019, my.clevelandclinic.org/health/articles/9118-female-reproductive-system.

“Gonadal Ridge.” Gonadal Ridge - an Overview | ScienceDirect Topics, www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/gonadal-ridge.

“Is a Pregnant Woman's Chance of Giving Birth to a Boy 50 Percent?” Scientific American, Scientific American, 15 Nov. 2004, www.scientificamerican.com/article/is-a-pregnant-womans-chan/.

“Female Age-Related Fertility Decline.” ACOG, Mar. 2014, www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2014/03/female-age-related-fertility-decline.

A, Aatsha P. “Embryology, Sexual Development.” StatPearls [Internet]., U.S. National Library of Medicine, 30 May 2020, www.ncbi.nlm.nih.gov/books/NBK557601/.

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