helluva lot of elective C-section and so choosing of 'convenient' birthdays like not-Christmas; not-4thJuly. But that won't allow anyone to shift large numbers of births between months. So the up-blip must be due to more successful tupping in the Christmas holidays. As you know, in human sexual relations it takes two to tango: so it might be that, with office-parties and drink-taken, there is more sexual activity. But I think it's also likely that conception is more likely to occur which results in a late Summer birthday - hormones are involved. That should surely be factored in for couples who a trying to conceive.
But there is no point in dreaming up an imaginative and internally consistent story to explain why we have such a situation, if the data shown above are an atypical aberration. First step is to see if the pattern is replicated with another dataset. That's what I'm pursuing this week with my hot-shot second-year strength & conditioning class. In Ireland the repository of statistical data is the Central Statistics Office aka Phríomh-Oifig Staidrimh and I hopped off there for a table of births by month for the six years 2007-2012 inclusive.
5006 is a February. Then, although these are substantial numbers you can smooth out some of the noise by adding up all the Januaries etc. between 2007-2012. Between us, we manged to agree that the handiest way of displaying the data was to sum up each month across years and then divide by the number of days in each month [See L]. This gives an average number of deliveries each day which is handy for planning how many midwives to employ to fill 3 shifts x 8 hours = 24hrs coverage. This comes down to about 200 births per day across the whole country. But they aren't uniformly distributed across the year and there does appear to be a July-Sept peak . . . except that August doesn't fit the trend. In the books Lies, Damned Lies and Statistics, Michael Wheeler has a whack at people exaggerating peaks in date by top-slicing the histogram as I've done here.
Brian Nosek and others insist on remaining calm when you get an interesting result . . . and repeating the experiment with a different, preferably larger, dataset. If you have Excel or similar, and the necessary curiosity, you might get equivalent data from the our country and send in a comment. One of the few things I remember from my own undergraduate career was taking part, as a cog, in a real-science experiment. I hope that there might be one student in my S&C class who will mention this week's exercise in 2035.