Neither boys nor girls are meaningfully harder to conceive. At the moment of fertilization, the split between male and female embryos is remarkably close to 50/50, with one major analysis estimating the proportion of males at conception at exactly 0.5. The slight boy surplus seen at birth, roughly 106 boys for every 100 girls in most populations, reflects not a conception bias but a complex chain of events during pregnancy itself. What makes sex determination so resistant to manipulation is that no reliably proven natural method can tip those odds in either direction.
What the Ratio Looks Like at Conception
This is one of the genuinely contested questions in reproductive biology. A 2015 analysis published in the Proceedings of the National Academy of Sciences used modeling of fetal mortality data across pregnancy to estimate the sex ratio at conception and concluded it was 0.5, meaning equal numbers of male and female embryos. That directly challenged the longstanding assumption that more males are conceived than females.1PubMed Central. The human sex ratio from conception to birth Other researchers disagree. A 2019 paper in Early Human Development argued that a substantial excess of males exists at conception, driven partly by parental hormone levels, and that the reason the birth ratio is only slightly male-biased is that male fetuses are disproportionately lost to miscarriage.2Early Human Development. The human sex ratio at conception
Data from assisted reproduction adds another layer. In IVF cycles, one study estimated the sex ratio at fertilization to fall between 1.29 and 1.50 (that is, 129 to 150 males per 100 females), while in ICSI cycles, where a single sperm is injected directly into the egg, the ratio dropped to about 1.07.3PubMed Central. Changes in sex ratio from fertilization to birth in assisted-reproductive-treatment cycles The disagreement hinges on how researchers model fetal loss at very early stages, when most miscarriages go undetected. Whether the starting ratio is truly equal or slightly male-heavy, though, the practical takeaway is the same: nothing you can do at the moment of conception gives you reliable control over which sperm reaches the egg first.
Why More Boys Are Born but More Boys Are Lost
Across most human populations, about 106 boys are born for every 100 girls. This well-documented pattern exists in part because male embryos and fetuses appear to be more vulnerable throughout pregnancy. A study of spontaneous abortions found the overall sex ratio among miscarried fetuses was 1.25, meaning males were significantly overrepresented. The male excess was confined to anatomically normal specimens and was present across gestational ages up to 23 weeks.4PubMed. Male excess among anatomically normal fetuses in spontaneous abortions
The reasons for this vulnerability are not fully understood, but the leading explanation involves the single X chromosome in male embryos. Female embryos carry two X chromosomes, which provides a backup copy of genes if one is faulty. Male embryos have one X and one Y, so a defective gene on the X chromosome has no redundant partner to compensate. This genetic asymmetry makes male embryos statistically more susceptible to developmental problems during pregnancy. So if more males are conceived (as some researchers argue), a larger share of them are also lost before birth, which narrows the gap to roughly 106 boys per 100 girls at delivery.
The Timing Myth
One of the most widely repeated pieces of folk advice about sex selection involves timing intercourse relative to ovulation. The idea, popularized by Landrum Shettles in the 1960s, holds that Y-bearing sperm (which produce boys) swim faster but die sooner, while X-bearing sperm (which produce girls) are slower but hardier. Under this theory, having sex closer to ovulation should favor boys, and having sex several days before should favor girls.
The problem is that the underlying premise has fallen apart. Modern studies using methods that reliably distinguish X-bearing from Y-bearing sperm found no evidence that Y-bearing sperm swim faster than X-bearing sperm.5Frontiers in Cell and Developmental Biology. New Biological Insights on X and Y Chromosome-Bearing Spermatozoa And when researchers tested the timing theory directly, it did not hold up. A study published in the New England Journal of Medicine tracked cycles producing male and female babies and found similar patterns of intercourse timing relative to ovulation. The authors concluded that the timing of sex has no practical influence on the sex of the baby.6PubMed. Timing of sexual intercourse in relation to ovulation. Effects on the probability of conception, survival of the pregnancy, and sex of the baby
Despite this, the Shettles method continues to circulate online and in fertility forums. Its persistence probably owes something to the fact that the outcome is always roughly 50/50 regardless of what you do, so anyone who tries it has about a coin-flip chance of “success” and may attribute that to the method.
Does Maternal Diet Nudge the Odds?
One of the more intriguing findings comes from research on maternal nutrition around the time of conception. A study of about 740 British women found that those in the highest third of caloric intake before pregnancy bore boys 56% of the time, compared with 45% in the lowest third. Women who ate breakfast cereal daily were also more likely to have boys.7PubMed Central. You are what your mother eats: evidence for maternal preconception diet influencing foetal sex in humans Animal research has shown even more dramatic effects: in mice, a very high-fat diet produced about 67% male pups, while a low-fat diet produced only 39% males.8PubMed Central. Striking variation in the sex ratio of pups born to mice according to whether maternal diet is high in fat or carbohydrate
The proposed mechanism relates to glucose levels. Higher blood glucose around conception may favor the survival of male embryos, which are slightly more metabolically demanding in their earliest divisions. But before anyone starts binge-eating cereal for a boy, the effect size in humans was modest, going from 45% to 56% is not exactly a reliable lever. And the mouse data, while striking, involved extreme dietary manipulation that doesn’t translate neatly to human meals. This is one of those findings where the biology is plausible and the direction is consistent, but the practical payoff for any individual couple is small enough that you could easily end up on the wrong side of the coin flip.
Does Having Boys or Girls Run in Families?
Many people believe that producing sons or daughters is somehow inherited, pointing to families with long runs of one sex. But a study that examined kinship correlations for the sex of children found no pattern consistent with genetic influence. If genes were driving the outcome, you would expect identical twins to show the strongest correlation in the sex of their offspring, followed by full siblings, then half-siblings, then cousins. Instead, the pattern was essentially the opposite: cousins showed a small positive correlation while identical twins showed none at all.9CHANCE. Does Having Boys or Girls Run in the Family?
Families with four or five sons in a row feel statistically remarkable, but they are not especially rare for a near-50/50 event repeated multiple times. With enough families in the world, long runs of one sex are guaranteed to appear purely by chance. The human tendency to notice patterns and assume a cause is doing most of the work here.
Parental Age and the Gap Between Partners
One demographic variable that does seem to correlate with the sex of offspring is the age difference between parents. A study examining this relationship found an inverted U-shaped association: the highest number of sons was observed when fathers were roughly six and a half years older than their partners. Interestingly, there was no significant relationship between parental age difference and the number of daughters.10PubMed. Age difference between parents influences parity and number of sons The mechanisms behind this are unclear, and the effect sizes are small enough that age difference is not a useful planning tool. But the finding does fit a broader picture in which male offspring seem more sensitive to parental and environmental conditions than female offspring are.
Environmental Exposures and Occupational Hazards
Certain chemical exposures have been linked to shifts in the birth sex ratio, though the evidence is patchy. A systematic review found that among paternal exposures, dioxins and PCBs were the most consistently associated with altered sex ratios. Dioxins were linked to fewer boys, while PCBs were linked to more boys. There was also limited evidence for decreased male births after paternal exposure to lead, methylmercury, and certain types of radiation. Few studies found clear effects from maternal exposures.11PubMed Central. Can environmental or occupational hazards alter the sex ratio at birth? A systematic review
A more recent population-based study examined 56 persistent organic pollutants and their association with the sex of offspring. After adjusting for multiple comparisons, only maternal exposure to mirex, an insecticide now banned in most countries, remained significantly associated with having more boys.12Environmental Research. Maternal and paternal serum concentrations of persistent organic pollutants and the secondary sex ratio: A population-based preconception cohort study The authors themselves described their findings as providing no conclusive evidence of an association. The chemicals that do show effects tend to be industrial contaminants that people are exposed to involuntarily, not factors anyone would choose to manipulate.
Smoking, another commonly discussed factor, does not appear to have a meaningful effect. A large prospective cohort study found no evidence of an association between smoking behavior around the time of conception and the sex of the resulting child.
Temperature, Season, and Climate
Several historical studies have found small correlations between ambient temperature and the sex ratio at birth. A study of 18th and 19th century Sami populations in northern Finland found that warmer years were associated with a slightly male-biased sex ratio. The net effect of a one-degree Celsius increase in annual mean temperature corresponded to roughly 1% more sons born that year.13PubMed Central. Temperature-related birth sex ratio bias in historical Sami: warm years bring more sons A study of 19th century records from the city of Poznań found a similar pattern, with higher temperatures around and before conception associated with a lower probability of girls being born.14Scientific Reports. Ambient temperature-related sex ratio at birth in historical urban populations: the example of the city of Poznań, 1848–1900
These effects are fascinating from a population-level perspective, and they may become relevant as climate change pushes average temperatures upward in many regions. But for an individual couple, a 1% shift is invisible. You would not notice it in a family of any plausible size.
The Returning Soldier Effect
One of the more curious patterns in demographic data is that more boys tend to be born during and immediately after major wars. This has been documented across both World Wars and is sometimes called the “returning soldier effect.”15PubMed. Big and tall soldiers are more likely to survive battle: a possible explanation for the ‘returning soldier effect’ on the secondary sex ratio The explanation remains debated. One hypothesis suggests that taller, larger men are more likely to survive battle, and the same traits associated with height (testosterone levels, for instance) may also be associated with fathering more sons. Another points to changes in how often couples have sex when partners are reunited after long separations. And a review of wartime sex ratio data noted that during the Iran-Iraq War, the pattern went in the opposite direction: the sex ratio shifted toward girls, possibly because psychological stress on pregnant women led to disproportionate loss of male fetuses.16Journal of Theoretical Biology. The variations of human sex ratio at birth during and after wars, and their potential explanations
The war data illustrate a broader principle: population-level shifts in the sex ratio can emerge from large environmental changes, but these shifts are tiny in absolute terms and driven by mechanisms that operate on entire societies, not on individual pregnancies.
The Trivers-Willard Hypothesis
Since the 1970s, evolutionary biologists have debated whether parents in better condition (wealthier, healthier, higher-status) should produce more sons, while parents in worse condition should produce more daughters. The logic is that sons have higher variance in reproductive success: a healthy, well-resourced son can father many offspring, but a disadvantaged son may father none. Daughters tend to have more consistent reproductive success regardless of conditions. This is known as the Trivers-Willard hypothesis.
A systematic review of five decades of research on this question in humans, covering 87 studies and over 800 individual tests, found that the overall evidence is consistent with the hypothesis. The estimated effect size was very small, on the order of a correlation of about 0.04.17Evolution and Human Behavior. Decades of Trivers-Willard research on humans: What conclusions can be drawn? In practical terms, that means the effect is real enough to detect across large datasets but so small that no individual couple could use it for prediction. The mechanism by which parental condition might influence fetal sex is still unclear and may involve differential survival of male embryos rather than any bias at conception itself.
Animal research has shown stronger effects. In a captive colony of rhesus monkeys studied over 20 years, high-ranking mothers were more likely to give birth to daughters while lower-ranking mothers were more likely to bear sons.18Nature. Birth sex ratios and social rank in rhesus monkey mothers The direction here is reversed from what a simple reading of Trivers-Willard would predict, since in rhesus monkeys, daughters of high-ranking females inherit their mother’s rank and thus benefit more from being born to a powerful mother than sons do. The finding illustrates how the same evolutionary logic can produce opposite outcomes depending on the social system of the species.
What Actually Works for Sex Selection
If natural methods are unreliable, technological ones are not. The most effective approach is preimplantation genetic testing during IVF, which identifies the sex of embryos before transfer and approaches 100% accuracy. For couples who want a less invasive route, sperm-sorting technologies have shown genuine efficacy. The MicroSort method, which uses fluorescent staining to separate X-bearing from Y-bearing sperm, can shift the natural 50/50 ratio to about 90% X-bearing or 75% Y-bearing sperm.19Reproductive BioMedicine Online. Scientific aspects of preconception gender selection A more recent clinical trial of a novel sperm separation technique also demonstrated significant enrichment of the desired sperm type and found that it did not impair fertilization rates or embryo development.20PLOS ONE. A non-randomized clinical trial to determine the safety and efficacy of a novel sperm sex selection technique
It is worth noting that the sex ratio can shift depending on which IVF technique is used even without intentional sex selection. Standard IVF is associated with a slightly higher proportion of male births compared to ICSI (about 54% versus 50%), and transferring blastocyst-stage embryos rather than earlier-stage embryos further increases the proportion of boys to nearly 59%.21PubMed Central. Factors affecting male-to-female ratio at birth in frozen-thawed embryo transfer cycles: a large retrospective cohort study This suggests that the lab environment itself introduces selection pressures that male and female embryos respond to differently, with male embryos apparently developing slightly faster to the blastocyst stage. Couples undergoing IVF for fertility reasons, not sex selection, may be surprised to learn that technical choices about embryo culture could nudge the odds.
Follicular Phase Length and Hormonal Windows
One lesser-known finding involves the length of the follicular phase, the stretch from the start of menstruation to ovulation. A study of 133 pregnancies found that shorter follicular phases (earlier ovulation) tended to produce boys, while longer follicular phases tended to produce girls. The mechanism is speculative but may relate to hormonal conditions in the uterine environment at the time the embryo implants. Women who ovulate earlier in their cycle have a different hormonal profile during the implantation window than women who ovulate later, and this could differentially affect male and female embryo survival.
This is a single study with a modest sample size, so it would be a mistake to treat it as settled science. But it aligns with the broader theme emerging from this research: the sex of the baby may be influenced less by which sperm reaches the egg and more by which embryos survive the first days and weeks after fertilization. The uterine environment, shaped by maternal hormones, nutrition, and immune function, appears to act as a filter that treats male and female embryos slightly differently.
Immune Tolerance and Male Pregnancies
One piece of the puzzle that receives less popular attention is the maternal immune system’s role in maintaining pregnancy. Because a male fetus carries Y-chromosome-derived proteins that the mother’s body has never encountered, pregnancies with male embryos require a more active immune accommodation. Research on regulatory T cells, which suppress immune rejection of the fetus, has found evidence suggesting that these cells need to be activated by male-specific antigens to be protective during pregnancies carrying boys.22PubMed. Regulatory T cells in pregnancy If this immune tolerance fails or is insufficient, the pregnancy is more likely to end in miscarriage. This adds yet another route through which male embryos face a slightly higher bar for survival than female embryos do, and it helps explain why male excess at conception (if it exists) gets trimmed down to the modest 106-to-100 ratio at birth.
The immune dimension also raises the question of whether prior pregnancies affect the sex ratio of later children. Some researchers have speculated that a mother’s immune system “remembers” male-specific antigens from a previous male pregnancy, potentially making it either easier or harder to carry a boy next time. The evidence on birth order and sex ratio is mixed and weak, but the immunological logic is at least coherent, and it underscores how many biological systems are quietly involved in something people tend to think of as pure chance.