Choosing Your Baby’s Sex: The Science

On your pregnancy journey, there’s no doubt you will hear many theories of how to select the sex of your baby. From timing intercourse for certain days of your cycle to separating the fast-swimming sperm (if you want a boy) from the slow-swimming sperm (if you want a girl), theories of sex-selection in utero abound. However, according to the science, the only sure-fire way of selecting the sex of your baby is by implanting an embryo with the sex chromosomes XY if you want a boy, or XX if you want a girl, in your uterus via in vitro fertilization (IVF).

In Vitro Fertilization

IVF is an expensive and intense treatment option that carries with it risks of ovarian hyperstimulation syndrome and higher instances of low birth-weight babies. It has traditionally been reserved for couples who have had trouble conceiving after trying for one and a half to two years. A weeks- to months-long process, IVF involves a series of hormone injections to stimulate a woman’s ovaries to produce multiple follicles (eggs) over a cycle. The follicles are then retrieved by way of a needle and fertilized with sperm in a lab. When sperm fertilizes an egg, the resulting product is a two-celled zygote (one cell from the sperm which carries either an X or a Y chromosome and one cell from the egg which always carries an X chromosome).

After maturing for a few days in the lab, the zygote becomes a six- to ten-celled embryo. The healthiest of the embryos is then implanted in a woman’s uterus in hopes that the pregnancy will become viable. Many times, more than one embryo is implanted. IVF success rates vary by the age of the mother and range from about 20% to about 50%.

Preimplantation Genetic Testing

Years ago, when I had IVF, the earliest we could get genetic testing done was at about 12 weeks’ gestation when we could get a procedure called a chorionic villus sampling (CVS). A CVS is a procedure in which a sample of the placenta is retrieved via a needle through the abdomen. Today, there is the much less invasive option of preimplantation genetic testing.

Preimplantation genetic testing is exactly what it sounds like. Scientists are able to take a sample of the embryo and identify genetic abnormalities BEFORE that embryo is actually implanted in a woman’s uterus during an IVF procedure. During that sampling, the chromosomes of the embryo are analyzed, including the chromosomes that determine sex.

Selecting Baby’s Sex

Why would someone want to select the sex of their baby? There are a few reasons. First, there are some diseases that run in families that are specific to sex. For example, Duchenne muscular dystrophy only affects males. Another reason is something people like to call ‘family balancing.’ Some families would like to have a certain ratio of male to female children.

In either case, after preimplantation genetic testing is done on an embryo that has been prepared for IVF, parents can select which embryo they’d like implanted in the mother’s uterus, either the one with the XX chromosomes or the one with the XY chromosomes.

Cost and Ethical Issues

First, the cost of IVF (in the range of $10,000 USD) may be prohibitive for some couples. Although some insurance may pay for the procedure if it’s deemed medically necessary, not all insurance will. Furthermore, as of 2018 there were 27.5 million uninsured people in the US for whom IVF would be totally cost-prohibitive. Add to that another $8,000 USD or so to cover the cost of preimplantation genetic testing.

Many worry that selecting the sex of baby raises ethical issues as well. Because of the cost, only the very rich would be able to take advantage of the sex selection procedure if it’s not medically necessary. There’s also the worry that such selection in society could lead to bias, or the preference of a child of one sex over the other, and an imbalance of the sexes in general.

A Word on Sex and Gender

These days, ‘sex’ and ‘gender’ are not words that can be used interchangeably. They mean two different things. Sex is the set of biological chromosomes a person is born with that make them male (XY) or female (XX). Gender is a more complication concept to explain. Gender is a societal construct. We prescribe gender norms based on whether a person is born male sex or female sex. Not all people who are born with XY will identify with the gender norms that have been traditionally assigned to them. Nor will all who are born with XX chromosomes.

It’s also important to remember that science is showing us that gender is not binary (either male or female). Gender can be seen as a spectrum and any person can fall anywhere along the spectrum. This can be determined by genetics, differences in hormone levels, or even differences in an area of the brain called the bed nucleus of the stria terminalis.


Perhaps, we need to look inside ourselves and ask why we would prefer to have a child of one sex over another. Is it to prevent disease? Is it because society tells us to value one and not the other? Is it because the parents just want to experience parenting a child with a certain set of chromosomes? Every child is different and every parenting experience is unique no matter what sex chromosomes a child has.

Given the cost, in the future, it could conceivably come about that sex selection of babies would be seen as a status symbol, like a designer purse. In fact, some celebrities with the financial means have already taken advantage of the option to select the sexes of their children.

Selecting the sex of baby is a very interesting and ethically problematic issue. Where do you stand?

Janette DeFelice
Dr. Janette DeFelice is a writer currently focusing on how the changing environment affects our health. She holds a Doctor of Medicine degree from Chicago Medical School where she taught clinical and diagnostic skills to beginning medical students, and a Master’s degree in Humanities from the University of Chicago. She also has a Bachelor’s degree in Political Science. Her writing can be seen online at BeTheChangeMom, ChicagoNow, and Medium, and she’s very excited to have published her first novel, Delia Rising: A Ballet in Three Acts. She lives in Chicago’s west suburbs with her school-age twins, her husband, and a family cat named Clara Barton.

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