The daily rhythms that shape our sleep, hormones and metabolism may begin to align with the outside world earlier than once thought.

New research suggests that a baby’s internal body clock can start synchronising with the mother’s daily cycle while still in the womb, with possible implications for pregnancy care and early development.

Humans and many other organisms run on circadian rhythms, roughly 24‑hour cycles that follow the pattern of day and night. These rhythms help regulate sleep and waking, body temperature, hormone release and energy use.

In adults, disruption to these rhythms is linked to health problems, including sleep disorders, metabolic disease and mood disorders.

In infants and children, early disturbances in sleep and daily rhythms have also been associated with later emotional and behavioural difficulties.

What has remained unclear is when the circadian system first becomes active during pregnancy and affect the development of the baby later on.

What the new research shows

A study published in the Journal of Biological Rhythms reports that circadian rhythms in mammals begin functioning before birth and later synchronise to the mother’s daily cycle.

The work was carried out in mice, not humans, and should be interpreted with appropriate caution.

Researchers at Washington University in St. Louis used genetically modified mice to track activity in foetal “clock” genes while the pups were still developing in the womb.

They found clear day–night patterns of clock gene activity in the foetuses during the final week of pregnancy. These rhythms became aligned with the mother’s own rest–activity cycle, a developmental stage roughly comparable to the third trimester in humans.

In plain terms, the study suggests that the basic machinery of the body clock forms early, and that timing signals from the mother later help set it to local time.

How it works

The developing foetus cannot detect light directly, so it relies on signals from the mother. The researchers found that synchronisation of the foetal clock coincided with the transfer of glucocorticoid hormones across the placenta.

Glucocorticoids are stress‑related hormones that naturally rise and fall across the day under the control of the mother’s circadian system. These rhythmic hormone signals appear to act as time cues, helping the foetal clock adjust to the mother’s daily pattern.

When pregnant mice were given synthetic glucocorticoids at regular intervals, the foetal clocks aligned with local time more quickly. This supports the idea that hormones, rather than light, are key messengers during pregnancy.

How strong is the evidence?

The findings are based on carefully designed animal experiments and provide strong, direct biological evidence that foetal circadian clocks can function before birth. However, the study was conducted in mice, and human development does not mirror mouse development in all respects.

While similar processes are likely to exist in people, the timing and clinical significance remain uncertain.

The authors also observed that foetuses lacking detectable clock activity were less likely to be delivered successfully, but it is not yet clear whether disrupted rhythms contribute to poor development or are simply a sign of it.

What this means for the public

For expectant parents, the research does not change current medical advice. There is no recommendation to alter sleep patterns, light exposure or daily routines based on this study alone.

However, the findings add to a growing body of evidence suggesting that stable daily rhythms during pregnancy may support healthy development.

Modern life often disrupts natural light–dark cycles, and most people are exposed to artificial light at night, including during pregnancy.

Treatment and clinical implications

One area of potential relevance is the use of synthetic glucocorticoids in pregnancy. These drugs are commonly given to women at risk of preterm birth to help mature the baby’s lungs.

The new study suggests that the timing of these medications could influence foetal circadian development, although this has not been tested in people.

Any changes to how or when such treatments are given would require careful clinical trials. For now, established medical practice remains unchanged.

What remains unknown

Key questions remain about when the human fetal clock becomes active, how strongly it responds to maternal signals, and whether disruptions during pregnancy have lasting effects on health.

Researchers are also exploring whether circadian development could serve as an early marker of foetal wellbeing.

Future studies in humans will be needed to determine how these animal findings translate to real‑world pregnancy care.

Closing perspective

The study adds an important piece to the puzzle of how biological timekeeping develops before birth. While the results are preliminary and based on animal research, they highlight pregnancy as a potentially sensitive period for baby’s body clock development.

Understanding how the body clock begins and how it is shaped may eventually help improve maternal care and long‑term health, but much remains to be learned.