What can effect if child is boy or girl
Executive summary
Biologically, which parent’s gamete supplies which sex chromosome is the main determinant: an egg always supplies X, while a sperm supplies X (girl) or Y (boy), so “the father” determines chromosomal sex at fertilization [1] [2]. However, researchers report additional genetic regulators and maternal or environmental factors can shift the odds modestly — studies point to gene enhancers, parental genetics, maternal blood pressure, timing within the menstrual cycle, seasonality and other correlates that may affect sex ratios, but none guarantee control over an individual pregnancy [3] [4] [5] [6] [7].
1. The basic biological rule: sperm supplies the deciding chromosome
Every human egg carries an X chromosome; sperm carry either X or Y, so fertilization by an X-bearing sperm yields XX (typically female) and by a Y-bearing sperm yields XY (typically male). Multiple mainstream summaries state that whichever chromosome the sperm carries determines the baby’s biological sex at conception [1] [2] [4].
2. Beyond X and Y: genetic switches and regulators
Modern genetics shows it’s not only the presence of X or Y that matters; gene regulatory elements control whether sex-development pathways are activated. Research from Murdoch Children’s Research Institute highlights a “regulator” that modulates genes like SOX9 involved in male/female development, meaning chromosomal status plus regulatory activity together produce the outcome [3].
3. Population-level shifts: fathers’ genes and inherited tendencies
Long-term family-tree studies suggest men may inherit tendencies to sire more sons or more daughters — implying some heritable component in sperm production or sex ratio bias. ScienceDaily summarizes work proposing that a man’s genes can subtly influence whether his sperm produce more X- or Y-bearing outcomes over many births [4].
4. Maternal physiology and small probabilistic influences
Epidemiological studies have identified maternal traits associated with slight changes in birth sex ratios. For example, one study found women who later delivered sons had marginally higher preconception systolic blood pressure (about 3 mmHg difference in that cohort), a correlation not fully explained and not a causal recipe for choosing sex [5]. Reviews of many studies note associations with menstrual cycle timing, diet, parental ages and environmental conditions — these are statistical signals, not reliable individual predictors [6] [8].
5. Timing and conception theories — what the evidence says
Methods like the Shettles timing theory and other attempts to influence sex by intercourse timing, diet or behavior have been widely discussed. Scientific reviews and Bayesian analyses show researchers repeatedly tested factors (follicular phase length, intercourse timing) and found only modest or inconsistent associations; the literature does not support confident control over sex by these means [6] [8].
6. Seasonality, environment and other population effects
Some datasets report that season, regional climate or large-scale environmental factors modestly shift sex ratios at birth (for instance, small month-to-month variations in birth counts), and some public tools extrapolate such patterns into prediction charts. But such population-level patterns vary between countries and decades and cannot guarantee a particular outcome for one couple [7] [9].
7. Medical testing and reliable determination versus influence
If the question is “how to know” rather than “how to affect,” medical tests can determine sex accurately: noninvasive prenatal testing (NIPT) can detect Y-chromosome fragments from about 10 weeks and ultrasound anatomy scans typically identify genitalia by mid-second trimester; preimplantation genetic testing in IVF determines sex almost certainly [10] [11] [12]. These are diagnostic — not methods to influence sex.
8. Takeaway: control is largely limited; odds can shift slightly
Available reporting shows chromosomal mechanics at fertilization are primary (egg = X; sperm = X or Y), and genetic regulators and parental or environmental factors can shift sex ratios a little across populations or families, but no routine, safe, non-assisted technique reliably guarantees a boy or girl for an individual pregnancy [1] [3] [4] [6]. If you want a definitive determination, medical tests provide high accuracy, but the ability to choose sex outside of IVF with preimplantation testing is not supported by strong, consistent evidence in natural conception [10] [11].
Limitations and disagreements to note: some sources present older or popular methods (timing charts, Chinese calendar) and claim seasonal or menstrual-cycle influences [7] [13] [9], while systematic scientific reviews emphasize inconsistent or small effects and caution against expecting reliable control [6] [8]. Available sources do not mention simple, proven behavioral actions that will guarantee one sex over the other in natural conception.