A scientific look at the urban myth that tidal forces on amniotic fluid cause more babies to be born during the full Moon.
Many labor and delivery doctors and nurses swear that more babies are born during the full Moon. Anecdotally, one often cited reason for this alleged effect is lunar tidal forces, the same effect that causes ocean tides.
How Lunar Tidal Forces Allegedly Cause Births
The reasoning behind the claim that tidal forces cause more births during the full Moon follows.
The Moon causes tidal forces acting on Earth’s oceans. Tidal forces are greater during the full Moon, producing more pronounced tides. Oceans are made of water. The amniotic fluid in a pregnant woman’s womb is mostly water. Lunar tidal forces therefore act on the amniotic fluid. During the full Moon, the greater tidal forces act on the amniotic fluid in the womb. The greater tidal forces stretch the woman’s womb causing her water to break, thereby inducing labor. Hence more babies are born during the full Moon.
Incorrect Physics and Flaws in the Reasoning
Major flaws in the above argument include:
- Tidal forces are also greater during the new Moon.
- Tidal forces on a woman’s womb are negligibly small.
To completely understand why the above line of reasoning is flawed, it helps to understand tidal effects.
In summary, tidal forces arise from the difference in gravitational forces acting on opposite sides of an object. The Moon’s gravity is strongest on the side of Earth closest to the Moon and weakest on the side of Earth opposite the Moon. The difference in gravitational force produces a tidal bulge in Earth’s oceans with high tides on the portions of Earth closest to and most distant from the Moon.
Tidal effects are greater during both the full and the new Moon because the Sun’s tidal effects add to the Moon’s tidal effects. The Sun’s tidal effects subtract from the Moon’s effects during the quarter Moon phases producing less pronounced tides.
Tidal forces are just as strong during the new Moon as they are during the full Moon. If the above reasoning were correct, there would be more babies born during both the full Moon and the new Moon. No one claims that more babies are born during the new Moon.
This point is however moot because the tidal forces on any object the size of a pregnant woman’s womb are infinitesimally small.
Mathematical Calculation of Tidal Force on a Pregnant Woman
As outlined in the article cited above, tidal forces are governed by the equation for the difference in the gravitational forces:
Here dF is the difference in the gravitational force between the two ends of the pregnant woman’s womb, which are a distance, dr, apart. The variable, r, represents the distance between the Moon, having a mass M, and the woman’s womb, with a mass m. G is the universal gravitational constant.
Assuming that a pregnant woman’s womb has a mass of about 10 kilograms and a diameter of about 0.3 meters and using standard values of the astronomical constants, I calculated dF for a pregnant woman’s womb.
The lunar tidal force on a womb is about 5X10^-13 newtons (=0.0000000000005 newtons). Solar tidal forces are even weaker. A newton is a unit of force that is roughly the weight of an apple.
Any woman who has ever given birth knows that a force equal to less than a trillionth the weight of an apple is not enough to push the baby out. Childbirth is much harder!
By way of comparison, the difference in lunar gravitational force between opposite sides of Earth is about 6X10^18 newtons(=6000000000000000000 newtons). Earth’s much larger size and mass causes this greater tidal force.
Statistical Analysis of Births and Lunar Phases
In science a good theory is nice, however experimental data can trump even the best theories. What do the data say about the frequency of births during the full Moon?
In the spring 1999 issue of IAPPP Communications, Dan Caton, an astronomy professor at Appalachian State University, and coworker, Pamela Wheatley examined the lunar phase during 50 million live births. They do not find an increase in birth rates during the full Moon. They in fact find a small unexplained increase in birth rates during the third quarter phase.
Lunar tidal forces can move oceans, but are much too weak, even during the full Moon, to induce labor. Despite what pregnant women may feel like, their wombs are just too small for tidal effects to be significant.
- Zeilik and Gregory, Introductory Astronomy & Astrophysics, Saunders, 1998. This Book provides mathematical details on tidal forces.
- Caton, D.B. and Wheatley, P.A. “Nativity and the Moon: Do Birthrates Depend on the Phases of the Moon?” International Amateur-Professional Photoelectric Photometry Communication, No. 74, pp. 50-54, Spring 1999.