If you're one of those super nice people who buys birthday presents for all your friends, then you have probably just had an expensive month.

There is also a strong possibility you've had your birthday recently or will do so in the next couple of days (in which case, happy birthday to you).

According to Statistics NZ, the 10 most common birthdays all appear in the 13-day period from September 22 to October 4 every year.

Out of this 13-day period, September 29 is the most common day for birthdays.

Darker shading indicates more common birthdays, lighter shading indicates less common birthdays. Photo / Statistics NZ

This data shows that more babies are conceived around the Christmas and New Years period than any other time of the year.

New Zealand is not the only country where this happens. According to Stats NZ, a very similar pattern can be found in birth data from Australia, England and Wales, and the United States (which means it can't also be related to summer).

The least common day of the year to have a birthday is December 25.

Statistics NZ reached this conclusion after analysing births data from 1980 to 2017.