Reproductive terms involved in sex determination

Before we get into how a baby is made., Llet’s start with some reproductive terminology to help paint a picture of what happens on a cellular level.

• Gamete – A gamete is a reproductive cell. Your body is made of a variety of different cells, like muscle cells, immune cells, and brain cells. Each cell has a specific function to help support your body’s health. A gamete’s job is to store your genetic information and use it to create a new life—a baby. There is one type of gamete from each of the two parents — male sperm and female ovum or egg. The sperm and ovum unite to create a baby.
• Sperm – The sperm is the male reproductive cell—also referred to as the male gamete. It holds the father’s DNA and fertilizes the egg to create an embryo, the cluster of cells that grow into a baby. Sperm cells are made from puberty, with new cells made daily.
• Ovum or Egg – The ovum is the female reproductive cell, or gamete. It stores the mother’s DNA and is stored and then released from the ovaries. The ovum is developed while the baby is in utero, and no more can be produced. They are dormant until puberty, and then begin to be released with each menstrual cycle.
• Embryo – An embryo is the combination of two cells—a male gamete and a female gamete. When a sperm fertilizes an egg, together, they create an embryo, the cluster of cells that will become a baby.
• Chromosome – The egg and the sperm both carry genetic information in the form of chromosomes. When an egg is fertilized, the male sperm provides half of an embryo’s chromosomes while the egg provides the other half to make a complete set of genetic information—enough to create a new human life.

What determines a baby’s sex?

Chromosomes determine everything from a baby’s eye color to shoe size. The pair of chromosomes responsible for gender determination of the offspring are called sex chromosomes. There are two kinds of sex chromosomes, X and Y. But what chromosome determines gender?

Women have XX chromosomes, that is, two X chromosomes, while men have XY sex chromosomes, that is, one of each kind. To complete the chromosome pair, a parent’s egg provides one sex chromosome-always an X chromosome-and the male sperm provides the other-which either an X or a Y chromosome.

If a male sperm carrying an X chromosome fertilizes the egg, it will combine with parent’s X, and the embryo will have two X chromosomes (XX), resulting in a female fetus. If a Y-carrying sperm combines with parent’s X, the resulting embryo will have one X and one Y chromosome (XY chromosome) thereby establishing a male fetus. Early testing helps identify the risk of chromosomal abnormalities such as an extra y Y chromosome (Jacob’s Syndrome) or extra Xx chromosome (Klinefelter Syndrome).

Accordingly, your baby’s sex is established at conception; however, up until 7-8 weeks, both male and female have identical genitalia from which reproductive organs develop. After 8 weeks, sex differences may be observable as fetal sex organs begin to develop and differentiate.

Who determines the sex of the baby?

Now that you know the “what,” you may be curious about the “who.” So, who determines the gender of the baby—as in, which parent? The answer is the father. A sperm cell can carry either of the two sex chromosomes, therefore it’s the male who determines the baby’s biological sex or given gender identity at birth.

You may wonder, are sperm cells equally likely to provide an X chromosome as they are to provide a Y chromosome?

Chances of having a girl vs. a boy

Recent research supports that sex selection is not equally likely. – In fact, Y chromosomal sperm is proportionally advantaged. Here are three studies that shed some light on why the newborn sex ratio may differ from the 50/50 answer from old wives’ tales:

• World Bank Birth Rate Data — According to the Pew Research Center, the global male-to-female birth ratio is around 1.07, meaning for every 100 girls, there are 107 boys born. While this varies by region, studies suggest that gender selection and social preferences for sons in certain countries, like those in Asia, contribute to this skewed ratio. In contrast, regions like sub-Saharan Africa experience lower male-to-female birth ratios, possibly due to higher miscarriage rates for male fetuses, especially in older women.
• The Family Tree Study — A genealogical study by Newcastle University analyzed over 1000 family trees and discovered that the sex of your child may be influenced by your family history. For instance, men with more brothers are more likely to have sons, while those with more sisters tend to have daughters. This is thought to be due to a gene that influences whether a man produces more X or Y chromosome sperm. If a man has two “m” genes, he’s more likely to have sons; with one “m” and one “f” gene, it’s a 50/50 chance, and with two “f” genes, he’s more likely to have daughters.
• War Time Births — A study on birth rates following wartime events found that there was an increase in male births after World War I. The research showed that returning soldiers were, on average, taller than those who didn’t survive, suggesting that taller parents are more likely to have sons. This phenomenon is linked to genetic and developmental advantages, leading to an increase in male births during times of reduced maternal stress, such as after the war. Compounded with the aforementioned statistic suggesting males are less likely to be born during high maternal stress levels, one may speculate the converse to be true – upon return from war, a woman’s stress may decrease, thereby increasing the likelihood of male births. You may also check out our gender test quiz for a fun way to explore what might determine your baby’s gender!

When can you tell the gender of your baby?

Wondering “when can you tell the gender of a baby?” but without an ultrasound scan? By 6 weeks into pregnancy, women will have detectable cell-free fetal DNA levels in their bloodstream. A simple gender blood test can screen this cell-free fetal DNA and look for Y chromosomes. SneakPeek Early Gender DNA Test works on the foundation that a parent has two X chromosomes. Detection of any Y chromosomes, in cell free fetal DNA means the parent is having a boy! If no Y chromosomes are detected, then the parents are having a little girl!

If you’re interested in discovering your baby’s sex sooner than ever before, you can take the test from the comfort of your home or at a local clinician’s office as early as 6 weeks into pregnancy.

It’s simple, safe, and best of all, accurate. The Early Gender DNA Test has proven to be over 99% accurate1 in laboratory studies.

Are you ready to know what your little one’s 23rd pair of chromosomes reveal?

FAQs on sex determination

Can you influence a baby’s sex?

While many pregnancy myths suggest ways to influence your baby’s sex, there is no scientifically proven method for guaranteed results. Factors like timing, diet, or positions have not been shown to significantly impact the chances of having a boy or girl. The only reliable method for gender selection is through medical interventions like IVF with preimplantation genetic testing (PGT), but this is typically reserved for medical reasons.⁸

How can you determine a baby’s gender?

There are several ways to determine a baby’s sex. The most common method is an ultrasound, typically done around 18-20 weeks, which can reveal the sex based on genital development. If you’re wondering how to know a baby’s gender without ultrasounds, a blood test like NIPT can provide gender information weeks prior to your appointment. The SneakPeek Early Gender Test offers a non-invasive, at-home test that can determine the baby’s gender as early as 6 weeks with over 99% accuracy, providing fast and reliable results before the traditional gender reveal ultrasound.

Is biological sex at birth random?

No, biological sex at birth is not entirely random. While sex chromosomes determine gender, studies show there may be influences that affect the likelihood of having a male or female child, including maternal factors and genetics. Some families exhibit patterns of having offspring of one sex more frequently, indicating that there may be underlying genetic or environmental factors influencing sex determination.⁹