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Are there really more births on full moons?

The lunar effect is the linking of everything from increased births and werewolf transformations to the changing phases of the moon.
The lunar effect is the linking of everything from increased births and werewolf transformations to the changing phases of the moon.
Photo courtesy of NPS

As the full moon nears, full-term pregnant women everywhere brace for delivery. After all, a full moon can kick them into labor -- right?

It's certainly a widely held belief. It goes back so far, it's unclear where it first took hold, but one thing is for sure: People around the world have a deep faith in the moon's ability to affect human physiology. Giving birth is just one human behavior said to be affected by the full moon. Conception and fertility have also been linked to moon phases; and then there are the entirely non-pregnancy related behaviors, like homicide rates, suicide rates, emergency room admissions and outbreaks of insanity. Supposedly, all of these things increase on a full moon. Oh, and don't forget werewolf-ism.

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It's called the lunar effect, and, as far as births are concerned, the primary explanation for the effect focuses on the moon's gravitational pull. It basically states that much the way the moon's gravity controls the tides, it can control a woman's body. The human body is 80 percent water, after all. And, given that both menstruation and ovulation roughly follow a lunar cycle -- occurring on a monthly basis -- it doesn't seem too far off to think that the moon could have a say in childbirth as well.

But does it? In this article, we'll take a look at some evidence for and against the lunar effect in birth rates, and find out if labor wards should be increasing their staff numbers every time there's a full moon. We'll also find out why so many people believe in the effect.

If you were to judge by word of mouth alone, it would seem as if the lunar effect was a sure thing. According to believers, one need only conduct a survey in a hospital to prove the connection between full moons and childbirth.

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Many labor nurses will tell you that their wards fill up on full moons.
Many labor nurses will tell you that their wards fill up on full moons.
Three Lions/Getty Images

If you ask around, you'll probably find that most people who believe in the lunar effect on birth rates rely on anecdotal proof: namely, stories related by medical professionals working in the obstetrics field. Labor nurses are often cited as providing evidence of the effect in the form of personal experience, basically relating that the number of labor-ward admissions increases on a full moon.

And who doesn't trust a labor nurse? Certainly, they're experts in the area of childbirth. But for a number of reasons that we'll get into on the next page, anecdotal evidence of this sort is notoriously unreliable.

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There is, however, a limited body of more empirical evidence for the existence of the phenomenon. A 1959 study broke the month into consecutive three-day periods and found that the three days of a "full moon window" -- the day before, day of and day after a full moon -- had more births than any other single three-day period [source: Shulman]. Another study, this one published in 1966, studied birth rates by moon phase -- full, half, one-quarter and three-quarter. The authors found that within the study period, more births centered around the full-moon phase than any other [source: Shulman].

But as you get deeper into the scientific study of the connection between birth and the full moon, it quickly becomes clear that these studies supporting the lunar effect are an anomaly. The vast majority of evidence reveals the lunar effect to be myth, not scientific reality.

And sometimes, studies seem to be misread by supporters of the belief. An article on the Web site BirthSource, for instance, makes the following statement:

Talk to nurses who work labor and delivery on a regular basis […] and they will all tell you that they believe in the power of the full moon. […] Another such study seems to verify this. Examined was the relationship between lunar position and the day of delivery […] The results showed a connection between spontaneous full-term deliveries and the lunar month. [source: BirthSource]

However, if you look at the abstract for the study the text refers to, [please note, in the source list, the article cites the study as reference "5," but in fact this text refers to reference "3."] you find the following statement:

[The effect of lunar phases on the time of delivery] is too weak to allow for prediction regarding the days with the highest frequency of deliveries. [source: ScienceDirect]

So, where does that leave us?

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Scientists will tell you there's no connection between births and moon phases.
Scientists will tell you there's no connection between births and moon phases.
Photo courtesy of NASA

Anecdotal and anomalous statistical evidence aside, it's tough to find proof that more babies are born on a full moon. The topic has been studied pretty extensively, though, and it's very easy to find evidence disproving the connection.

Here are just a handful of the scientific studies that have found no connection between the full moon and birth rates:

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1957: Looked at a series of days with abnormally high numbers of births and tried to correlate them with full moons. No correlation was found [source: Shulman].

1987: Looked at U.S. birth rates by decade and found no correlation between full moons, birth rates or conception rates [source: Shulman].

1996: Looked at 100 previous studies on lunar effects and found no statistically significant proof of the moon's effect on birth, violence, suicides, major disasters or a dozen other supposedly lunar-connected phenomena [source: Skeptic's Dictionary].

1998: Looked at 3,706 births and found that "scientific analysis of data does not support the belief that the number of births increases as the full moon approaches, therefore it is a myth not reality" [source: ScienceDirect].

2005: Looked at 564,039 births in North Carolina between 1997 and 2001 and found "no predictable influence of the lunar cycle on deliveries or complications" [source: PubMed].

2006: Looked at births over a 28-year period in Australia and found that "full moons are not associated with any significant change in the number of conceptions, births, or deaths" [source: Gans].

If there's so much evidence against the lunar effect, why is the belief so widespread? For pretty much the same reasons why lots of other old wives' tales or urban legends pick up a strong following: It's easy to believe.

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Cognitive bias is one reason why the full moon birth myth has prevailed.
Cognitive bias is one reason why the full moon birth myth has prevailed.
©iStockphoto.com/vividpixels

The belief that the number of births increases on a full moon is a longstanding one, and one with cultural roots. Folkloric tales have, by definition, been around for long enough to seem like common sense. It can be counterintuitive to argue with a belief that has been passed down through countless generations.

It can also be hard to argue with experts like labor nurses and paramedics. Who would know better?

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And then there's the media's love of the spooky old full moon. The press loves a good human interest story about the full moon filling up labor wards or causing a paramedic shortage; and Hollywood loves the iconic werewolf-howling-at-the-full-moon image.

Cognitive bias plays a strong role, as well. Cognitive bias is a psychological phenomenon in which people absorb all evidence that supports their belief and ignore all evidence to the contrary. In this case, that could mean a nurse noticing every time the ward is full on a full moon, but not noticing every time it's empty on a full moon.

And finally, there are some simple misconceptions regarding certain aspects of the moon. For instance, a full moon really only lasts an instant; a "full moon window" of three days, or even one day, is an artificial construct [source: Shulman]. Also, many people believe the moon's gravity is extremely powerful, since it affects the tides; in fact, it's a relatively weak gravitational force, and one unlikely to have an effect on the human body [source: Skeptic's Dictionary]. And the gravitational force that effects the tides is not even phase-related; it's a function of how far the moon is from Earth at any given time, which is different from the phase the moon is in [source: Skeptic's Dictionary].

But hey, all is not lost. We can still hang on to one of the best lunar-effect beliefs: There's no hard evidence that the moon doesn't affect werewolves.

For more information on the lunar effect and related topics, look over the links on the next page.

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Related HowStuffWorks Articles

Sources

  • Arliss JM, Kaplan EN, Galvin SL. "The effect of the lunar cycle on frequency of births and birth complications." Am J Obstet Gynecol. 2005 May; 192(5):1462-4. Pubmed. http://www.ncbi.nlm.nih.gov/pubmed/15902138
  • Full Moon, Gravitational Pull and Childbirth! BirthSource. http://www.birthsource.com/Scripts/article.asp?articleid=409
  • Full Moon and Lunar Effects. The Skeptic's Dictionary. Feb. 23, 2009. http://www.skepdic.com/fullmoon.html
  • Gans, Joshua S. and Andrew Leigh. "Does the Lunar Cycle Affect Birth and Deaths?" The Australian National University Centre for Economic Policy Research. August 2006. http://econrsss.anu.edu.au/pdf/DP532.pdf
  • Joshi, Raksha, et al. "Labor ward workload waxes and wanes with the lunar cycle, myth or reality?" ScienceDirect. May 31, 2000.http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TBM-40CS1W8-2W&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&_docanchor=&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=6afe02a25a52cdfc8c2b267a85201460
  • Shulman, Holly B. "Investigating Lunar Cycles in Monthly Fertility Rates." Bureau of the Census. April 29, 1987. http://www.census.gov/srd/papers/pdf/rr87-12.pdf

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