Progesterone does not determine your baby’s sex. A baby’s sex is locked in at the moment of fertilization, depending entirely on whether the sperm that reaches the egg carries an X chromosome (girl) or a Y chromosome (boy). No hormone you take or naturally produce can change that outcome after conception. However, a small but growing body of research suggests that progesterone levels may have a subtle statistical association with sex ratios, particularly in IVF settings, even if the effect is far too small to function as any kind of gender selection tool.
How Sex Is Actually Determined
The chromosomal sex of an embryo is established the instant a sperm fertilizes an egg. Every egg carries an X chromosome. Sperm carry either an X or a Y. If a Y-bearing sperm wins the race, the result is a male embryo (XY). If an X-bearing sperm arrives first, the result is a female embryo (XX). This is a one-time event that no hormone can reverse or override.
For roughly the first six weeks after conception, male and female embryos look identical at the level of their developing reproductive organs. The gonads sit in a “bipotential” state, meaning they could become either testes or ovaries. After week six, genetic signals from the sex chromosomes push the gonads down one path or the other. If testes form, they drive male development of the internal and external genitalia regardless of the surrounding hormone environment. If no testes form, the body follows the female developmental pathway by default. Progesterone circulating in the mother’s bloodstream doesn’t change which path the embryo takes.
What Progesterone Does During Conception
Progesterone plays a real role in the biology of fertilization, just not in the way many people hope. Lab research has shown that progesterone is a chemical attractant for sperm. It’s the main steroid in follicular fluid (the liquid surrounding the egg at ovulation), and it helps guide sperm toward the egg through a process called chemotaxis. Among several hormones tested, progesterone was the only one that consistently drew sperm in dose-response experiments.
The key question is whether progesterone attracts X-bearing and Y-bearing sperm differently. So far, no study has demonstrated a reliable selective effect. Both types of sperm respond to the progesterone signal. While it’s biologically plausible that subtle differences in sperm behavior could exist, there is no proven mechanism by which your progesterone levels could filter for one type of sperm over the other in a way that meaningfully shifts the odds.
The IVF Data: A Small Statistical Signal
One of the more interesting findings comes from IVF research. A study of 424 births following frozen embryo transfer found that when maternal progesterone levels exceeded 20 ng/mL, the odds of having a girl rose by about 38% compared to women with lower progesterone levels. Conversely, higher progesterone was associated with 28% lower odds of a male birth. These results were statistically significant, though just barely.
Before reading too much into those numbers, context matters. The overall sex ratio in the study was almost perfectly even: 49.8% girls and 50.2% boys. The 38% increase in odds sounds dramatic, but it translates to a shift of roughly 5 to 15 percentage points in the probability of a girl across different age and weight combinations. That’s a statistical pattern detectable in a large dataset, not a reliable predictor for any individual pregnancy. Maternal BMI also played a role in the model, and the study used frozen embryo transfers, which involve a very specific hormonal environment that doesn’t mirror natural conception.
Natural Progesterone Levels and Baby Sex
Studies measuring progesterone in naturally conceived pregnancies have produced mixed results. A large Finnish study analyzing blood samples from over 1,300 women during the first half of pregnancy found that after week 12, progesterone levels were about 6% lower in women carrying girls compared to those carrying boys. But the researchers themselves described the findings as “inconclusive,” noting that other studies found no difference at all.
This is an important distinction: even where a correlation exists, it likely reflects the pregnancy responding to the fetus rather than the mother’s hormones determining fetal sex. A male fetus with developing testes produces its own hormonal signals that can subtly influence the mother’s hormone profile. In other words, the baby’s sex may be nudging progesterone levels, not the other way around. One Scandinavian study found a similar pattern, with progesterone running slightly lower in the second trimester for women carrying girls, while estrogen was about 9% higher in those pregnancies.
Why the Old Wives’ Tales Persist
The idea that your hormone levels reveal or influence your baby’s sex feeds a whole category of pregnancy folklore. Girls supposedly steal your beauty. Boys make your hair grow faster. A moodier pregnancy means a girl. These beliefs stem from the reasonable intuition that male and female fetuses would create noticeably different hormonal environments in the mother.
The evidence doesn’t support this. The hormone differences that researchers can detect between boy and girl pregnancies are small, inconsistent across studies, and only visible when averaging large groups of women. They are not large enough to produce the dramatic symptoms people attribute to them. Any individual woman’s progesterone or estrogen level is shaped far more by her own body, her placenta, and the stage of pregnancy than by fetal sex.
Can Progesterone Supplements Shift the Odds?
Many women take progesterone supplements during pregnancy, especially in IVF cycles or to prevent preterm birth. There is no evidence that taking supplemental progesterone will make you more likely to have a boy or a girl. The IVF study showing a statistical association involved measured blood levels of progesterone in a controlled clinical setting, not a simple cause-and-effect relationship between taking a supplement and getting a particular outcome.
Safety research on progesterone use in pregnancy has looked at whether it affects fetal genital development. Some older studies raised a concern about a possible link between certain synthetic progestins and hypospadias (a minor structural difference in male genitalia), but those studies had methodological problems. Most well-designed studies since then have not found a higher rate of birth defects in children whose mothers took progesterone or progestins during pregnancy. Progesterone supplements support pregnancy, but they don’t reprogram fetal sex development.
The Evolutionary Theory Behind Sex Ratio Shifts
There is a well-known evolutionary hypothesis, proposed by biologists Robert Trivers and Dan Willard, that predicts mothers in better physical condition should produce more sons, while mothers in poorer condition should produce more daughters. The logic: in species where males compete for mates, a healthy, well-resourced son has a disproportionate reproductive advantage, while a daughter’s reproductive success is more stable regardless of condition.
Some researchers have tried to connect this theory to maternal hormone levels, including progesterone and testosterone, as potential mechanisms. The theory is intellectually interesting, but the actual shifts in human sex ratios linked to maternal condition are tiny. Humans consistently produce boys and girls at close to a 50/50 ratio across nearly all measured conditions. Whatever fine-tuning mechanisms might exist in mammals, they don’t give individual parents a meaningful lever to pull.
The bottom line: progesterone is essential for a healthy pregnancy, and there are faint statistical associations between progesterone levels and offspring sex in research settings. But these associations are small, inconsistent, and not something you can use to influence whether you have a boy or a girl. Sex is determined by sperm, at fertilization, in a fraction of a second.