Published on August 27th, 2020 and Updated on February 29th, 2024
Table of Contents
What Determines the Sex of a Baby?
Who Determines the Sex of the Baby?
When Can You Tell the Gender of Your Baby?
Are we having a boy or a girl? This is the common question many parents ask soon after finding out they are pregnant; historically, answers suggest fetal sex prediction has equal probability. Today, studies suggest there is more to this story. Continue to learn more about what determines the gender of a baby, what genes are passed down from the father only to determine the sex of a baby, as well as how to use a sneak peek test.
How A Baby’s Gender is Determined: Unlocking the Gender Mystery
We first begin with how a baby is made. Let’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.
How does gender fit into this picture?
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,– an X and a Y. To complete the chromosome pair, a mom-to-be’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 mom’s X, and the embryo will have two X chromosomes (XX) resulting in a female fetus. If a Y-carrying sperm combines with mom’s X, the resulting embryo will have one X and one Y chromosome (XY chromosome) thereby establishing a male fetus. Early testing helps identify risk of chromosomal abnormalities such as an extra y chromosome (Jacob’s Syndrome)or extra x 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 difference maybe observable as fetal sex organs begin to develop and differentiate.
Who Determines the Sex of the Baby?
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?
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
The Pew Research Center investigated the World Bank birth rate data to determine differences in the global sex ratios of males to females. They found the male-to-female birth ratio was 1.07. Meaning there are 107 male births for every 100 female births. Even in countries with the lowest male-to-female birth ratios, there is still a statistically significant disproportionate share of boys to girls.
One leading theory for high global male-to-female ratio is gender selection, or sex-selective abortion in Asia. Social and economic pressures appear to have led to fewer female births in countries where sons are preferred. Alternatively, a lower sex ratio associated with sub-Saharan African countries, while still male heavy, may be due in part that the likelihood of miscarriage in boys increases in geriatric pregnancies (older pregnant women), and in regions where intercourse occurs less often. Maternal stress also increases miscarriage rates and is more likely for male fetuses.
The Family Tree Study
In a large genealogical study, Newcastle University researchers analyzed nearly 1000 family trees dating back to 1600, with over half a million people total.!
Investigation indicates an undiscovered gene may control whether a male contains more Y chromosome sperm or more X chromosome sperm. Through their model, they observed the following:
“The family tree study showed that whether you’re likely to have a boy or a girl is inherited. We now know that men are more likely to have sons if they have more brothers but are more likely to have daughters if they have more sisters. However, in women, you just can’t predict it.”
The researchers speculate that the creation of sperm cells is likely to be determined by two types of genes. In a simplified scenario:
- If a male has 2 “m” genes, he is more likely to produce Y chromosome sperm and have sons.
- If a male has 1 “m” and 1 “f” gene, he is equally likely to produce either X or Y sperm.
- If a male has 2 “f” genes, he is more likely to produce X chromosome sperm and have daughters.
War Time Births
Another study observed increased male births during and after war times. After World War I, the heights of soldiers returning from combat were compared with those who didn’t survive. Turns out, the average height of returning soldiers was nearly an inch taller than those who had fallen (about 66.4 inches to 65.5 inches).
Satoshi Kanazawa explains that taller parents are more likely to have sons. As taller people survived, likely due to genetic and developmental advantages, more male births occurred thereafter. This small 1-inch difference across a generation of soldier-parents is enough to explain millions of excess baby boy births than girl births in the UK post WWI. 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 likelihood of male births.
Conclusion: Are There Different Likelihoods of Having a Boy or Girl?
Yes – While the chances may be close to 50-50, evidence suggests that you can look toward the male’s family history as the dominant indicator.
The one way to know for sure? An early gender DNA test.
When Can You Tell the Gender of Your Baby?
Wondering how to know your babies gender 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 mom has two X chromosomes,. Detection of any Y chromosomes, in cell free fetal DNA means she’s having a boy! If no Y chromosomes are detected, then she’s 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 weeks7 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?
Sources:
Palomar.edu Recombination and Linkage. https://www2.palomar.edu/anthro/biobasis/bio_3.htm
University of Rochester Medical Center. X-linked Recessive: Red-Green Color Blindness, Hemophilia A. https://www.urmc.rochester.edu/encyclopedia/content.aspx?ContentTypeID=90&ContentID=P02164
Pew Research Center. The odds that you will give birth to a boy or girl depend on where in the world you live.
The odds that you will give birth to a boy or girl depend on where in the world you live
Trends in Population Sex Ratios May be Explained by Changes in the Frequencies of Polymorphic Alleles of a Sex Ratio Gene https://link.springer.com/article/10.1007/s11692-008-9046-3
Psychology Today. The Returning Soldier Effect I: Why Are More Boys Born During and After Wars? https://www.psychologytoday.com/us/blog/the-scientific-fundamentalist/200802/the-returning-soldier-effect-i-why-are-more-boys-born
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SneakPeek aims to provide the most accurate and up-to-date information to help our readers make informed decisions regarding their health before, during, and after pregnancy. This article was written based upon trusted scientific research studies and/or articles. Credible information sources for this article are cited and hyperlinked.
