Gender Selection Explained: How Sex Selection Before Pregnancy Works

In short: The most accurate way to choose the baby’s sex is preimplantation genetic diagnosis (PGD) with IVF, which is 99–100% accurate. Insemination after sperm separation is about 80% accurate, while diet and timing of intercourse are much less reliable. Sex selection is of particular value for sex-linked inherited diseases.

For many hopeful parents, the question of whether their baby will be a boy or a girl has always been fascinating. Throughout history, families have followed cultural traditions and folk beliefs to try to influence their baby’s sex. In some cases, there is also a genuine medical reason — such as an inherited condition linked to one sex — that makes gender selection medically relevant. In this article, we walk through the history of these beliefs, the science behind sex determination, and the modern methods available today, including their approximate reported success rates.

How a Baby’s Sex Is Determined

Since the 1980s, researchers have studied how a baby’s sex is determined biologically. It comes down to the type of chromosome carried by the sperm — either an X-chromosome or a Y-chromosome. The egg (oocyte) always carries an X-chromosome. So when an X-sperm fertilizes the egg, the result is a girl (XX); when a Y-sperm fertilizes the egg, the result is a boy (XY).

A Look Back: Historical Beliefs About Choosing a Baby’s Sex

Long before modern genetics, many cultures had their own theories. The ancient Greeks believed the Y-sperm came from the right testis and the X-sperm from the left, so some men tied off the left testis hoping to avoid having a daughter. Similarly, some Indian traditions held that pressing the left testis during intercourse would favor a son. In France, some men even underwent surgical removal of the left testis for the same belief.

Other folk beliefs suggested that a fat man and thin woman would only have daughters (and vice versa), or that eating spicy foods, meats, salted fish, or animal organs would produce sons. Some also believed the day of the week for intercourse influenced the baby’s sex. None of these beliefs have scientific support, but they show just how long people have wondered about this question.

Below, we look at methods that have been studied for those hoping to influence the chances of having a boy or a girl — from simple lifestyle approaches to advanced laboratory techniques.

Diet-Based Approaches to Gender Selection

Some researchers, including Rajan S. Joshi, have explored whether a mother’s diet may influence the chemical environment of the egg’s outer wall, potentially affecting which type of sperm is more likely to fertilize it.

The theory suggests that the balance of sodium and potassium versus calcium and magnesium in the diet may affect the receptors on the egg wall.

According to this theory, a diet higher in sodium and potassium, and lower in calcium and magnesium, may create conditions that favor the Y-sperm — potentially increasing the chances of a boy.

Conversely, a diet richer in calcium and magnesium, with less sodium and potassium, may favor the X-sperm — potentially increasing the chances of a girl.

Couples interested in trying this approach are typically advised to follow the specific diet for at least two months before attempting to conceive.

It is important to know that this diet theory has not been proven in good-quality scientific studies. A very salty diet, or cutting out calcium-rich foods before pregnancy, is not healthy; any diet should stay balanced, and folic acid should be taken before conception.

Timing Intercourse to Influence Baby’s Sex

This approach is based on the theory that Y-sperm are lighter and faster-moving but shorter-lived, while X-sperm are heavier, slower, and longer-lived.

Based on this theory, timing intercourse close to ovulation was thought to favor a boy and intercourse several days before ovulation to favor a girl; however, careful scientific studies have found that timing does not change the chance of having a boy or a girl, and X- and Y-sperm do not differ meaningfully in speed or survival.

For couples wanting to combine approaches, pairing this timing method with the diet described above — and precisely tracking ovulation, which can vary from person to person and cycle to cycle — may be considered, always under medical guidance.

Vaginal pH and Gender Selection

Some theories suggest that a more acidic vaginal environment favors the X-sperm, while a more alkaline environment favors the Y-sperm. Vaginal douches have been used to try to shift this balance, but there is no reliable evidence that they influence the baby’s sex, and douching is not recommended because it can disturb the natural vaginal flora and increase the risk of infection.

These combined approaches are traditional and have not been shown to work in scientific studies; douching is not advised because of the risk of infection.

Sperm Sorting (XY Separation) with IUI for Gender Selection

In this approach, the ovaries are stimulated with medication to encourage the development of multiple follicles and improve the chances of pregnancy. The sperm sample is then processed and sorted in the laboratory before being inseminated into the uterus (IUI).

It’s important for couples to understand that this method is not fully accurate, so pregnancy with the non-preferred sex can still occur.

Combining this method with the recommended diet and precise ovulation timing may help improve the odds, though results can vary.

Several older sperm-sorting techniques have been tried — based on sperm weight, electrical activity, or passing sperm through albumin — but these are generally considered unreliable by today’s standards.

Flow Cytometry Sperm Sorting for Gender Selection

A more advanced laboratory method, known as flow cytometry sperm sorting, separates sperm based on their DNA content — X-sperm contain about 2.8% more DNA than Y-sperm. Using a flow cytometer, and confirming results with the FISH technique (which colors Y-sperm green and X-sperm pink-red), this method has been reported to sort Y-sperm with around 73% accuracy and X-sperm with around 88% accuracy.

A typical semen sample naturally contains roughly equal numbers of X- and Y-sperm, though this can vary.

The sorted sample can then be used for IUI, IVF, or ICSI, with combined success rates for achieving the desired sex reported as high in some published data.

This technique is only available in a limited number of specialized centers worldwide.

Preimplantation Genetic Diagnosis (PGD) for Gender Selection

PGD is currently considered the most accurate method available, with reported accuracy close to 99-100%. Before considering this option, it’s important to discuss several factors with your fertility specialist:

1- The mother’s age.

2- The number and sex of any existing children.

3- The mother’s overall health and delivery history.

4- Whether the couple feels prepared for the possibility of a pregnancy with the non-preferred sex, since no method is entirely without exceptions.

The PGD process generally involves the following steps:

Step 1 — Ovarian stimulation: Hormonal injections are used to stimulate the ovaries, with follicle growth monitored by ultrasound until they reach the appropriate size.

Step 2 — Egg retrieval and fertilization: Eggs are retrieved from the ovaries, typically under sedation or general anesthesia, and fertilized with sperm in the laboratory under a microscope.

(Patients sometimes ask whether sorting X- and Y-sperm before fertilization could increase the number of embryos of the desired sex. Current evidence suggests this pre-sorting step does not make a meaningful difference when PGD is already being used.)

Step 3 — Embryo culture and biopsy: Embryos are cultured in the laboratory for five to six days until they reach the blastocyst stage. A small number of cells are then carefully removed from each embryo for genetic and chromosomal analysis to determine its sex.

Step 4 — Embryo transfer: Healthy embryos of the desired sex are selected for transfer into the uterus. In some cases, no embryos may be suitable for transfer if they are chromosomally abnormal or not of the preferred sex.

Step 5 — Pregnancy test: After transfer, patients typically continue supportive medication and wait about two weeks before taking a pregnancy test. While PGD is highly accurate for sex selection and is not associated with an increased risk of birth defects, some studies suggest pregnancy rates may be somewhat lower compared to standard IVF cycles that do not include sex selection.

The Chinese Gender Calendar

This traditional calendar, said to date back roughly 700 years, is one of the oldest known methods for predicting or influencing a baby’s sex.

It’s based on the mother’s age and the month of conception, associated with the traditional elements of water, earth, wood, fire, and metal. It has no scientific basis: a large study of more than two million births found that its predictions were correct only about 50% of the time — the same as chance.

We’ve included the traditional chart below for reference:

Across the top: the month the child was conceived.

Down the side: the mother’s age at the time of conception.

Comparing Reported Success Rates

Natural conception without following any method: approximately 51% boys, 49% girls.

Diet, timing, douching and the Chinese calendar are unproven claims with no scientific support. The table keeps PGD (about 99%) and the natural baseline (about 51%). In some countries, sex selection using PGD is restricted by law unless it is medically necessary due to a sex-linked genetic condition. Regulations vary by country, so it’s worth checking local guidelines or discussing this with your fertility specialist.

Frequently Asked Questions

1. Does gender selection affect the baby’s health? These methods are not known to negatively affect the baby’s health.

2. Is any method linked to birth defects or miscarriage? Current evidence does not associate these methods with increased risk of birth defects or miscarriage.

3. Can these methods be guaranteed? No method other than PGD (or, less reliably, sperm sorting) has been shown to change the chance of a boy or a girl.

4. Is my privacy protected during these procedures? Yes — patient confidentiality is a priority throughout diagnosis and treatment.

5. When can I find out my baby’s sex? A blood test (NIPT) can show the baby’s sex from about 10 weeks, and ultrasound can reliably confirm it at around 18–20 weeks; earlier scans are less accurate.

Chromosomal Sex Determination Video

Chromosomal Sex Determination Video

If you’re considering gender selection for family balancing or medical reasons, it’s best to discuss your specific situation, health history, and local regulations with a qualified fertility specialist. Dr. Najeeb Layyous and his team welcome the opportunity to answer your questions and guide you through the options available.

Dr Najeeb Layyous F.R.C.O.G

Consultant Obstetrician, Gynecologist and Infertility Specialist