Full moon, more babies?

When I came through the doors to Labor & Delivery for one of my last births in South Bend, a nurse I’d seen not very long before, for a different client, smiled at me as she rushed by.

“Full moons, right?”

I grinned back at her, repeating, “Full moons.”

That’s the only time someone has ever actually directly invoked to me this very common (superstition? belief?) idea in maternity units: more babies are born during the full moon than at other points in the lunar cycle. But it’s definitely the topic of discussion in many birth circles. When I brought it up at dinner one night, my husband and my best friend *laughed* at me. Then teased me about how absolutely unscientific this sounded.

“That’s ridiculous!”

“Please tell me that’s not a thing you actually believe, right?”

Again, I grinned and repeated, “Not a thing I actually believe.” But, I kept thinking about this superstition.

So, that night, I started digging into the academic research on lunar moon phases and birth frequency.

This not only exists; it is EXTENSIVE.

And I’m going to walk you through a little bit of it now.

Let’s start off with a 2005 study published in the American Journal of Obstetrics and Gynecology, which looked at the effect of the lunar cycle on the number of births (both total and to folks who had already given birth), on the percentage of cesarean births, and on complications (Arliss et al 2005). This study included over 560,000 births in North Carolina that occurred across 62 lunar cycles. They found NO EFFECT of lunar cycles on those outcomes I mentioned.

Several other studies have born this out too (pun intended). (See Morton-Pradhan, Bay, & Coonrod 2005 and Staboulidou et al 2009 for examples, and see Kardong-Edgren 1995 for an overview of literature prior to that point.)

They all find the same thing: no correlation between number of births and the lunar cycle.

So much for the full moon theory, right?

NOT SO FAST.

In 2020, a new article was published by Matsumoto and Shirhashi, who had a brilliant thought (and a small sample size, to be very fair, but the fact that they found what they did given their lower statistical power may actually be a point in their favor — I digress, but come take a statistics class with me if you’re curious).

Instead of looking at the number of births across lunar cycles, they decided to look at the number of daytime vs nighttime births across lunar cycles.

No one had done this before.

And guess what? The numbers were different across the lunar cycle!!

More nighttime babies are born around the full moon, while more daytime babies are born around the new moon.

What’s even cooler is that when they looked at the same sample but used the approach that other scholars had already used (looking just at total number of births per day across the lunar cycle), they didn’t find any correlation between births and the moon. This means that their finding demonstrates that prior research was not getting the wrong answer but was asking the wrong question. (I love methodological discoveries like that!)

The authors give several possible explanations for why moonlight vs daylight might affect birth numbers, and for why this myth of more babies on full moons has come to be, including:

• maybe natural onset of labor has to do with circadian rhythms and darkness,
• maybe full moons are more noticeable so birthing women and medical teams pay more attention to the moon’s state during its fullness,
• maybe lunar phases or moonlight levels can affect oxytocin synthesis,
• maybe (like some other mammals) our melatonin rates have something to do with labor,
• maybe it’s actually about darkness facilitating birth and the moon is just one possible source of light,
• etc.

I find this absolutely fascinating. If you do too, and you want to read more, you can find the article here.

So all those L&D workers who believe that their night shifts are always busier during full moons…might actually be correct!

It’s just not because more babies are born those days; it might be that more babies are born those nights.

Works Cited:

Arliss, Jill M., Erin N. Kaplan, and Shelley L. Galvin. 2005. “The Effect of the Lunar Cycle on Frequency of Births and Birth Complications.” American Journal of Obstetrics and Gynecology 192(5):1462–64. doi: 10.1016/j.ajog.2004.12.034.

Kardong-Edgren, Suzan. 1995. “Full Moon, Barometric Pressure, and Human Birth.” The Journal of Perinatal Education 4(1):21–25.

Matsumoto, Shin-ichiro, and Kiyohiko Shirahashi. 2020. “Novel Perspectives on the Influence of the Lunar Cycle on the Timing of Full-Term Human Births.” Chronobiology International 37(7):1082–89. doi: 10.1080/07420528.2020.1785485.

Morton-Pradhan, Susan, R. Curtis Bay, and Dean V. Coonrod. 2005. “Birth Rate and Its Correlation with the Lunar Cycle and Specific Atmospheric Conditions.” American Journal of Obstetrics & Gynecology 192(6):1970–73. doi: 10.1016/j.ajog.2005.02.066.

Staboulidou, Ismini, Philipp Soergel, Bernhard Vaske, and Peter Hillemanns. 2008. “The Influence of Lunar Cycle on Frequency of Birth, Birth Complications, Neonatal Outcome and the Gender: A Retrospective Analysis.” Acta Obstetricia et Gynecologica Scandinavica 87(8):875–79. doi: 10.1080/00016340802233090.