Are you more likely to give birth on a full moon?

I have been attending ante-natal classes recently and, although I greatly appreciated our teacher, I found myself skeptical about some of her claims.  For example, our instructor was adamant that it is more likely to give birth during a full moon than on other days. She even told us that maternity wards are over-run during full moon nights!

In order to investigate this claim, I decided to look at some birth records. The centre for disease control in the US keeps very thorough records of births. These can be freely downloaded from: http://www.cdc.gov/nchs/data_access/Vitalstatsonline.htm . It is also possible to download the date for full moons from this website .

Having obtained data for dates of birth and dates of full moon, I  wrote a program to extract the probability of giving birth as a function of phase in the moon. The following graph shows the results for births in the US in 1969.

The x axis shows the number of days between the birth and the full moon and the y axis shows the probability of birth. If there was a correlation between full moon and likelihood of giving birth, there should be a peak in the distribution about x=0. However, clearly, the distribution is flat. Therefore there is no correlation between these two quantities. I guess it is still possible that only british women feel the influence of the moon. I somehow doubt that!

Here is the code I used to analyse the data:


import bisect
#birth data from CDC: http://www.cdc.gov/nchs/data_access/Vitalstatsonline.htm
#Phases of the moon from http://moonphases.info/past_full_moon_dates_calendar.html#1970

codeToYear={'9':1969, '0':1970, '1':1971}


FullMoon1969 = [ [03,01,1969], [02,02,1969], [04,03,1969], [02,04,1969], [02,05,1969], [31,05,1969], [29,06,1969], \
[29,07,1969], [27, 8,1969], [25, 9,1969], [25,10,1969], [23,11,1969], [23,12,1969] ]

#here i build the cumulative number of days from the beginning of a year to a certain month, in order to convert a date
#into an integer describing the # of the day of the year
length_of_month = [ 31, 28, 31, 30, 31,30,31,31,30,31, 30,31]
cum_day_by_month = dict()
cum = 0;
for i in range(1, 13):
cum_day_by_month[i] = cum
cum += length_of_month[i-1]

def get_birth_day(s):
year = codeToYear[s[0]]
month = int(s[83:85])
day   = int(s[85:87])
return [ day, month, year]

def hash_date(date):
[d,m,y] = date
# when the day is not recored, the field is entered with a 99
if d != 99 :
return cum_day_by_month[m] + d
else :
return - 1

def get_hash_birth_day(s):
return hash_date(get_birth_day(s))

# this list contains the day of the year when a full moon occured
full_moon_hashed = map(lambda x : hash_date(x) , FullMoon1969)

def d_from_full_moon(h):
# bisect_left ensures that h <= full_moon[i]
i = bisect.bisect_left(full_moon_hashed, h)
if i != len(full_moon_hashed) :
d1 = h-full_moon_hashed[i]
d2 = h-full_moon_hashed[i-1]
if abs(d1) < abs(d2) :
return d1
else:
return d2
else:
return h-full_moon_hashed[i-1]


distribution_full_moon_d = list()
for i in range(36):
distribution_full_moon_d.append(0)

f = open("NATL1969.PUB", 'r')
count = 0
for line in f:
hd = get_hash_birth_day(line)
#ignore the births where the date is non-reported
if hd != -1 :
d = d_from_full_moon(hd)
distribution_full_moon_d[d+18] += 1
count += 1

print "#number of records counted: " , count
print "#days from/to full moon, probability"
for d in range(36):
print d-18, float(distribution_full_moon_d[d])/count


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About jakirkpatrick

I am a researcher in solar energy at the University of Oxford. I am interested in mathematics, programming and trying to understand why things work. I also like the great outdoors and riding my bike.
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5 Responses to Are you more likely to give birth on a full moon?

  1. Roland says:

    Hm, looking at the data I do see an increase between -1 and +9 days around full moon. That doesn’t mean it’s significant. You don’t say anything about the underlying sample sizes but I assume they are pretty high. Why not do a test?

  2. H Dog says:

    What about solar storms?

  3. RatBag says:

    Agreeing with Roland. That slight excess between -2 and +9 probably _is_ enough to overflow a maternity ward in NHS! Significance test, pretty please.

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