Women’s sense of smell is roughly 50% more sensitive than men’s on average, and the gap widens during pregnancy to a degree that some evolutionary biologists think is a defence mechanism for the fetus – Image for illustrative purposes only (Image credits: Pixabay)
Walk into any well-equipped smell laboratory and the pattern emerges quickly: women, on average, pick up faint odors at concentrations that leave many men still searching. The difference is not dramatic in every single test, yet it holds across repeated studies and different cultures. What stands out is how sharply that edge grows once pregnancy begins.
Measuring the Everyday Gap
Researchers assess smell ability through several standard tasks. One measures the weakest concentration at which an odor becomes detectable. Another asks participants to name a scent from a short list. A third checks whether two similar smells can be told apart. Women tend to score higher on all three, though the size of the advantage shifts with the specific odor, the time of month, and the age of those taking part.
Cell counts in the olfactory bulb offer one clue, yet they do not tell the whole story. Even when researchers account for differences in bulb size, women still perform better. That points to another layer of influence that changes with hormonal state rather than fixed anatomy alone.
How Reproductive Hormones Tune Sensitivity
Oestrogen and progesterone act directly on the receptors that line the nasal passages and on the brain areas that interpret their signals. Sensitivity rises and falls across the menstrual cycle, reaching its highest point near ovulation when oestrogen peaks. After menopause, when oestrogen levels drop, the advantage over men of the same age narrows noticeably. The same hormonal pathways that shape other sensory experiences appear to be at work here as well.
Pregnancy takes this modulation to an extreme. Circulating oestrogen can climb to roughly one hundred times its usual level by the third trimester, while progesterone also reaches heights seen only during gestation. If ordinary cycle changes already alter smell perception, the far larger hormonal surge of pregnancy should produce a correspondingly larger shift. Controlled tests have indeed recorded modest but measurable gains in detection thresholds during the first trimester, especially for food-related and spoilage-related odors.
Pregnancy Sickness and the Defence Idea
The timing of these changes lines up with the period when the developing embryo is most vulnerable to chemical harm. Morning sickness, including strong aversions to certain smells and foods, peaks in the same early weeks when organs are forming. One long-standing explanation holds that the heightened sensitivity helps steer the mother away from foods that might carry plant toxins or bacterial risks. The pattern of aversions observed in studies – meat, fish, eggs, and strong vegetables – matches the categories that would have posed the greatest threat across evolutionary time.
Support for this view comes from the way the changes cluster around the exact window of greatest fetal risk. Yet not every researcher accepts a purpose-built defence story. A 2025 analysis from UCLA researchers instead links the same symptoms more closely to the body’s inflammatory response to the placenta and fetus. In this reading, the smell changes may be a side effect of immune adjustments rather than a system shaped specifically for protection. The two accounts are not entirely at odds; an inflammatory process could still produce useful avoidance behaviour even if that was not its original function.
Recent work on the placenta itself adds further context. An international team based at the University of Vienna traced the specialised cells at the maternal-fetal boundary back roughly 100 million years. Their findings portray pregnancy as a long-running negotiation in which both mother and fetus have adapted over deep time to balance nutrition and safety. Heightened smell sensitivity fits comfortably inside that broader picture of layered adjustments, even while the precise evolutionary pathway remains under discussion.
Where the Evidence Stops Short
The effect is real, yet several stronger claims do not hold up under scrutiny. Pregnant women do not suddenly detect odors that are literally undetectable to everyone else; the shift is quantitative rather than qualitative. What changes most is how unpleasant a given smell feels. Individual experience also varies widely: some women notice little or no increase, while others report reduced sensitivity later in pregnancy. The averaged picture is consistent, but personal outcomes differ.
Whether the entire system evolved expressly to shield the fetus or whether it simply happens to be useful remains an open question. The hormonal timing, the pattern of aversions, and the period of fetal vulnerability all line up with a protective role. At the same time, competing explanations centered on inflammation continue to receive serious attention. The mechanism itself is well documented; the ultimate purpose is still debated.
Key points to keep in mind
- Women show higher average sensitivity on standard smell tests, with the gap widening during pregnancy.
- Hormonal changes, especially rising oestrogen, appear to drive much of the difference.
- The changes coincide with the most vulnerable stage of fetal development, though whether this reflects deliberate evolutionary design is unresolved.
- Individual results vary, and stronger claims about undetectable odors lack support.
Put together, the findings describe a measurable biological difference shaped by reproductive hormones and amplified during pregnancy. The result resembles a defence system in its timing and effects, yet researchers continue to weigh whether that resemblance reflects selection or coincidence. The numbers are clear; the evolutionary interpretation is still being refined.
Hi, I’m Andrew, and I come from India. Experienced content specialist with a passion for writing. My forte includes health and wellness, Travel, Animals, and Nature. A nature nomad, I am obsessed with mountains and love high-altitude trekking. I have been on several Himalayan treks in India including the Everest Base Camp in Nepal, a profound experience.
