Have you ever heard of someone say, “Wow, the emergency room was hopping last night.  It must have been a full moon.” Or “If your due date comes close to a full moon, that is when you’ll have the baby.”  Is this documented and evidence-based or simply Urban Legend?

While it is very unlikely that we as humans know all that there is to know about medicine and science, some studies do show that SOMETHING is going on.  Dr. Leiber's study (J Clin Psychiatry. 1978. May 39(5): 385-92)⁶, postulates "the existence of a biological rhythm of human aggression which resonates with the lunar synodic cycle." 

Does that explain the erratic behavior of people during a full moon?  Probably.  Does it answer our question about childbirth and full moon?

Is there any relationship between the times when babies are born and the lunar cycle?

There are published works that show that there is such a relationship. One study⁴ looked at 5,927,978 French births occurring between the months of January 1968 and the 31st December 1974. Using spectral analysis, it was shown that there are two different rhythms in birth frequencies: --a weekly rhythm characterized by the lowest number of births on a Sunday and the largest number on a Tuesday and an annual rhythm with the maximum number of births in May and the minimum in September-October. A statistical analysis of the distribution of births in the lunar month shows that more are born between the last quarter and the new moon, and fewer are born in the first quarter of the moon. The differences between the distribution observed during the lunar month and the theoretical distribution are statistically significant.

People are 80% water.  And the changes in barometric pressure changes tides…does that change how we act or react?  Some say yes, others no.  Those that say no site the power of folklore/tradition/urban legends, misconceptions and cognitive biases (all bad events don't occur during the full moon and all events that occur during the full moon aren't bad!).  Talk to nurses who work labor and delivery on a regular basis, midwives,  busy doulas or experienced childbirth educators and they will all tell you that they believe in the power of the full moon plus changes in barometric pressure from cold/warm fronts (but that is another article!)

Another such study seems to verify this⁵.  Examined was the relationship between lunar position and the day of delivery and the synodic (in astronomy, length of time during which a body in the solar system makes one orbit of the sun relative to the earth) distribution of spontaneous deliveries, especially in relation to the presence of a full moon. A retrospective analysis of 1248 spontaneous full-term deliveries in three-year period (36 lunar months) was done at the Department of Obstetrics and Gynaecology, Civil Hospital, Fano (Marche, Italy). The results showed a connection between spontaneous full-term deliveries and the lunar month. The effect of the phases of the moon seems to be particularly relevant in mothers who had birthed before.

References:

1. Boklage CE. Effects of a behavioural rhythm on conception probability and pregnancy outcome. Human Reproduction. 1996 Oct;11(10):2276-84. 
2. Criss TB, Marcum JP.  A lunar effect on fertility. Social Biology. 1981 Spring-Summer;28(1-2):75-80.
3. Ghiandoni G, Secli R, Rocchi MB, Ugolini G.Does lunar position influence the time of delivery? A statistical analysis. European Journal of Obstetrics, Gynecology and Reproductive Biology. 1998 Mar;77(1):47-50.
4. Guillon P, Guillon D, Lansac J, Soutoul JH, Bertrand P, Hornecker JP.  Births, fertility, rhythms and lunar cycle. A statistical study of 5,927,978 births Journal of Obstetrics, Gynecology and Reproductive Biology. (Paris). 1986;15(3):265-71
5. Joshi R, Bharadwaj A, Gallousis S, Matthews R.Labor ward workload waxes and wanes with the lunar cycle, myth or reality? Primary Care Update Ob Gyns. 1998 Jul 1;5(4):184.
6. Leiber, A.L. Human aggression and the lunar synodic cycle. Journal of Clinical Psychiatry. 1978. May 39(5): 385-92).
7. Periti E, Biagiotti R.  Lunar phases and incidence of spontaneous deliveries. Our experience. Minerva Ginecol. 1994 Jul-Aug;46(7-8):429-33.