What Determines If You’ll Have a Boy or Girl?

Whether a baby turns out to be a boy or a girl comes down to which sperm fertilizes the egg: one carrying an X chromosome produces a girl, while one carrying a Y chromosome produces a boy. That part of the story is straightforward, and it has been understood for over a century. What makes the question far more interesting is everything that happens around that coin-flip moment, because the odds are not perfectly even, and dozens of biological and environmental factors appear to nudge the ratio in one direction or the other. None of these nudges is large enough to let you reliably choose, but together they explain some genuinely surprising patterns in who ends up with sons versus daughters.

The Sperm Decide, but They Are Nearly Identical

A common belief holds that Y-bearing sperm swim faster while X-bearing sperm are sturdier and live longer. This idea, popularized in the 1960s and 1970s, became the foundation for timing-based sex-selection methods. Newer research paints a very different picture. A 2019 review found that X- and Y-bearing sperm show negligible or no differences in ratio, shape, size, motility, swimming pattern, or stress response; the only established difference is their DNA content, with X sperm carrying slightly more genetic material.¹ There is one subtle surface-level distinction: X-bearing sperm carry more sialic acid on their cell membranes, giving them a slightly higher net negative charge, a finding confirmed through laboratory electrophoresis.² Whether this charge difference matters in the chaotic environment of the reproductive tract is another question entirely, and there is no strong evidence that it does.

The Ratio at Conception Is Close to 50-50

People sometimes hear that more boys are conceived than girls. That claim turns out to be shakier than it sounds. A large analysis published in the Proceedings of the National Academy of Sciences estimated the sex ratio at conception at 0.5, meaning equal numbers of males and females.³ What happens after conception is where things get complicated. Male embryos are more likely to die in the first week or two, tipping the ratio slightly toward girls. Then female mortality rises for the next several months, swinging it back toward boys. By the time babies are born, the worldwide ratio settles at roughly 105 boys for every 100 girls. That slight male excess at birth is not because more boys are conceived; it is because the pattern of pregnancy loss differs between the sexes at different stages.

An analysis of large birth datasets confirmed that the male-to-female ratio at birth hovers near 1.05 and does not change meaningfully with either maternal or paternal age.⁴ In other words, being older or younger when you have children does not shift your odds of having a boy versus a girl.

Why Timing Methods Do Not Work

The Shettles method, one of the most widely circulated sex-selection folk techniques, advises couples to time intercourse relative to ovulation: have sex closer to ovulation for a boy, a few days before for a girl. The reasoning relies on the assumption that Y-bearing sperm are faster but die sooner. A study published in the New England Journal of Medicine tracked the timing of intercourse relative to ovulation and found that cycles producing boys and girls had similar patterns; for practical purposes, the timing of sex has no influence on the baby’s sex.⁵ A separate four-year study that specifically tested a modified version of Shettles’ approach in 73 women found no statistically significant difference from the sex ratios produced by uncontrolled intercourse.⁶

The persistence of the Shettles method in popular culture is one of the more frustrating corners of reproductive science. It sounds plausible and feels actionable, which is exactly why it keeps circulating, even though the underlying premise about sperm speed has been contradicted by modern evidence. Any website or book still recommending it is working from outdated biology.

Maternal Diet and the Calorie Connection

One of the more provocative findings in sex-ratio research comes from diet studies. In a study of 740 British women, those in the top third of calorie intake around the time of conception had a 56% chance of bearing a boy, compared with 45% for women in the lowest third.⁷ That is a noticeable difference, though still far from a guarantee of either sex. The same general pattern has been observed in mice, where the effect was more dramatic: mothers fed a very-high-fat diet produced litters that were about two-thirds male, while mothers on a low-fat diet produced litters that were only about 39% male.⁸

The mechanism behind this is not settled, but one hypothesis points to glucose availability around the time of conception influencing which embryos survive the earliest stages. Male embryos may be more sensitive to metabolic conditions, thriving when energy is abundant and failing more often when it is scarce. This fits a broader evolutionary framework suggesting that mothers in good condition tend to invest in sons, whose reproductive payoff is higher when they are healthy and competitive.

Hormones in the Follicle

Another intriguing line of research focuses on the hormonal environment of the egg before fertilization. In cattle, ova that developed in follicular fluid with high testosterone levels were significantly more likely to be fertilized by Y-bearing sperm.⁹ This finding supports what has been called the “maternal dominance” hypothesis: before conception, higher follicular testosterone might influence the egg so that it is already adapted to receive one type of sperm over the other.¹⁰

In humans, a related observation came from tracking ovulation timing. Among 133 pregnancies carried to term, conception cycles with a short follicular phase (early ovulation) tended to produce boys, while longer follicular phases tended to produce girls.¹¹ Because follicular-phase length is influenced by hormonal levels, this could be another facet of the same testosterone-related mechanism. These findings are suggestive rather than definitive, and nobody has turned them into a reliable method for choosing a baby’s sex, but they do point toward the mother’s body playing an active role in something long assumed to be entirely random.

Stress, Disasters, and the Fragile Male Fetus

A pattern documented across several countries is that the proportion of boys born tends to dip after major catastrophic events. Earthquakes, famines, and other acute stressors have been linked to a temporary decline in the male-to-female birth ratio, possibly because male fetuses are lost at higher rates three to five months after the stressor or because fewer males are conceived in the months following the event.¹² One proposed pathway involves stress hormones, particularly androgens from the mother’s adrenal glands, which could selectively harm male pregnancies.¹³

However, the evidence is not as clean as it sounds. A systematic review found that while 16 studies reported a decline in the male-to-female ratio after catastrophic events, three found a rise, four found no change, and two gave contradictory results.¹⁴ The inconsistency likely reflects differences in how “stress” was measured, how population data were analyzed, and whether confounding variables were controlled. The general direction of the evidence points toward stress lowering the share of boys born, but the effect is small and unreliable enough that any given disaster may or may not produce a measurable shift.

Father’s Family Tree and a Possible Genetic Tendency

If you have ever noticed that some families seem to produce mostly boys or mostly girls across generations, you are not imagining things. Research by Corry Gellatly, using family tree data, found that men inherit a tendency to have more sons or more daughters from their parents. A man with many brothers is more likely to father sons, while a man with many sisters is more likely to father daughters.¹⁵ The proposed explanation involves a gene (or genes) that influence the proportion of X- versus Y-bearing sperm a man produces. If such a gene exists, it would not be detectable through standard sperm analysis, because the overall ratio is still close to 50-50; the skew would be subtle, perhaps 52-48 or 48-52, but enough to tilt the odds over multiple children.

This hypothesis has an appealing evolutionary logic: during wartime, when many men die, the few surviving fathers who carry a “more sons” gene produce more boys, who then carry the same gene, which helps replenish the male population. The model is theoretical and not yet confirmed at the molecular level, but it offers a tidy explanation for one of the better-documented curiosities in sex-ratio data.

The Returning Soldier Effect and Wartime Birth Ratios

During and immediately after both World Wars, multiple countries recorded a small but consistent rise in the proportion of boys born. Several explanations have been proposed, including changes in how often couples had sex (more frequent intercourse after long separations) and selective survival on the battlefield. One analysis argued that taller soldiers were more likely to survive combat, and taller parents are statistically more likely to have sons; conservative estimates suggest that a one-inch height advantage among surviving soldiers could more than account for the excess boys born in the UK during and after World War I.¹⁶ Other researchers have noted that both the height hypothesis and the coital-rate hypothesis could contribute simultaneously.¹⁷ The wartime birth-ratio shift is real but small, and untangling its causes from a distance of decades is challenging.

Toxins, Smoking, and Temperature

Environmental exposures can also nudge the sex ratio. A systematic review of occupational and environmental hazards found that paternal exposure to dioxins was consistently associated with fewer boys being born, while exposure to polychlorinated biphenyls (PCBs) was associated with more boys.¹⁸ The reasons are poorly understood but may involve selective damage to one type of sperm or differential survival of embryos. Smoking is another factor: when both parents smoked heavily (more than 20 cigarettes a day), the proportion of male offspring was significantly lower than among non-smoking couples.¹⁹

Temperature has drawn attention as well. Studies using historical data from Scandinavian and Finnish populations found that warmer years were associated with a higher proportion of boys born, while colder years slightly favored girls.²⁰ A separate analysis of Nordic birth cohorts from the late 1800s and early 1900s similarly found that cold temperatures during gestation predicted lower sex ratios.²¹ More recently, a study of 19th-century births in the city of Poznań confirmed a nearly linear relationship: the higher the temperature in the year of conception, the more likely the baby was to be a boy.²² The effect sizes are small, on the order of about 1% more sons per degree Celsius of warming, and the biological pathway remains unclear. One possibility is that temperature-related stress selectively affects male or female embryos during vulnerable windows of early development.

How Fertility Treatments Shift the Odds

Couples undergoing assisted reproduction may be surprised to learn that the type of fertility treatment they use can subtly change the sex ratio of their babies. Standard IVF (mixing egg and sperm in a dish) tends to produce a slight male bias, while ICSI (injecting a single sperm directly into the egg) tips the ratio slightly toward girls. A large analysis found the proportion of males born was about 52% after IVF but close to 50% or just below after ICSI.²³ Another study with a large sample confirmed a similar pattern: ICSI produced a sex ratio of 50.0%, compared with 52.2% for IVF and 52.2% for IUI or ovulation induction.²⁴

When frozen embryos are transferred, the stage at which the embryo was frozen also matters. Blastocyst-stage transfers (day five or six) are associated with a higher proportion of male offspring compared with cleavage-stage transfers (day two or three), with one large retrospective study reporting roughly 59% boys after blastocyst transfer versus about 52% after cleavage transfer.²⁵ Male embryos may develop slightly faster, making them more likely to reach the blastocyst stage and be selected for transfer. None of these shifts is large enough to serve as a sex-selection strategy on its own, but they are consistent and well-documented enough that fertility clinics are aware of them.

For couples who specifically want to choose, sperm-sorting technology offers the closest thing to a reliable method outside of preimplantation genetic testing. The MicroSort process, which separates sperm by DNA content using a fluorescent dye, can shift the natural 50-50 ratio to roughly 88% X-bearing (girl) or 74% Y-bearing (boy) sperm after sorting.²⁶ Among babies actually born using this technology, roughly 93% were girls after X-sorting and 85% were boys after Y-sorting. It is not perfect, and it requires assisted reproduction procedures like IUI or IVF, but it is far more effective than any natural-timing or diet-based method.

When the Usual Rules Break Down

The standard model of sex determination treats the Y chromosome’s SRY gene as the master switch: if it is present, the embryo develops male gonads; if not, female development proceeds. But rare cases show this is an oversimplification. A case report described a 46,XX individual (chromosomally female) who carried the SRY gene on a derivative X chromosome yet developed with typical female external genitalia throughout pregnancy and after birth.²⁷ Conversely, people with 46,XY chromosomes can develop female anatomy if the SRY gene is deleted or non-functional, or if downstream signaling pathways are disrupted. These differences in sex development, classified by chromosomal constitution, illustrate that sex determination involves a cascade of genetic events, not a single binary switch.²⁸

In most other vertebrates, the picture is even more varied. Many reptile species use temperature-dependent sex determination, where the heat of the nest during a critical window decides whether embryos become male or female. Mammals, including humans, rely on genetic sex determination instead, but the existence of temperature-dependent systems in close evolutionary relatives is a reminder that sex determination is not the same across the animal kingdom. In reptiles with this system, both male and female development are active processes requiring the simultaneous activation and suppression of competing genetic pathways, rather than one sex being the “default.”²⁹

Sex-Selective Practices and the Human Element

Biology is only part of the story when it comes to the sex ratio of actual births around the world. In several countries, cultural son preference combined with access to prenatal sex determination has skewed birth ratios far beyond anything biology can explain. Vietnam, for instance, has maintained an abnormally high sex ratio at birth for more than two decades, driven largely by sex-selective abortion, with the strongest skew in the country’s northern regions where son preference is most entrenched.³⁰ Similar patterns have been documented among certain migrant communities. A study of births in Australia found that Indian- and Chinese-born mothers had elevated rates of induced abortion in early pregnancy that coincided with the introduction of non-invasive prenatal sex testing, providing indirect evidence of sex-selective abortion practices contributing to male-biased birth ratios in those populations.³¹

These human interventions dwarf any of the biological effects discussed earlier. Diet, stress, temperature, and sperm genetics might shift the ratio by a percentage point or two. Sex-selective abortion can push it to 115 or 120 boys per 100 girls in affected populations, creating demographic imbalances with long-term social consequences. When people ask what determines whether a baby is a boy or a girl, the honest answer includes not just chromosomes and hormones but the cultural and technological environment the pregnancy occurs in.