Some hospital workers say that more babies are born on full-moon nights. Is it true? In the United States, researchers looked at more than 564,000 births. They put the births into groups by the shape of the moon.

The number of births was almost the same in every group. A study in Germany with over four million births found the same thing. The moon moves the sea, but it does not choose a baby's birthday.

Many people, including some hospital staff, believe that more babies are born at full moon. Researchers tested this idea with birth records. A 2005 study analyzed 564,039 births in North Carolina and sorted them into eight phases of the lunar cycle. It found no meaningful difference in the number of births or in birth complications.

A German team later examined more than four million births using a method that searches data for repeating cycles. They found a weekly pattern and a yearly pattern, but no cycle of about 29.5 days, the length of the lunar cycle. Although the moon's gravity causes ocean tides, its effect on something as small as a human body is tiny. The belief probably survives because busy full-moon nights are easier to remember.

On some maternity wards, staff will tell you that the full moon brings a rush of births. The belief sounds reasonable enough. If the moon can lift the oceans, why not affect a pregnant body? Large datasets say otherwise. Arliss and colleagues sorted 564,039 births in North Carolina into eight lunar phases and found no meaningful difference in either the number of births or the rate of complications.

A German team went further. They applied spectral analysis, a technique for detecting hidden repeating patterns, to more than four million births recorded over nearly four decades. It picked up a weekly cycle, linked to planned deliveries, and a seasonal cycle.

Nothing matched the 29.5-day lunar month. Physics explains why. Tides happen because the moon's pull differs slightly across the enormous width of the Earth's oceans. Across a human body less than two meters tall, that difference is negligible. One study did find something.

A French analysis of 38.7 million births detected a slight rise at full moon. With that many births, even a tiny difference can pass a statistical test, and this rise was too faint for anyone on a ward to notice.