Childhood Diseases

Posted by: Shelby | 2009-12-04


Maths Question

Hi Doctor,

I have a maths question for you. My son was born on the 12th July, which is a Sunday. I have been counting the following Sundays as him being 1 week older, 2 weeks older, etc, for the purpose of his vaccinations. He was therefore 20 weeks on the 29th November, which is 5 months, but he will only be 5 calendar months on the 12th December, if I count from the 12th of every month.

I am confused when to take him for his 9 month measles injection, will it be 36 weeks on the 22nd March or 9 months on 12th April?

Hope you get what I am trying to ask!

Expert's Reply



Your maths problem is caused by the fact that there are either 30 or 31 days in most months. If you take a month at 4 weeks you have either 2 or 3 days over for each month.This is roughly an extra week every 2 to 3 months. It does not really matter. Work on the months from now on for your baby's next immunisations.He should be 9 months old when he has his measles immunisation.

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user comments


Posted by: Anon | 2009-12-04

I have been told to count a babies age by the weeks till they are about 12-16 weeks and then you go by the months. It will not really matter if he got his measles injection on the 22nd of March or 12 April. He does not need to be exactly 9 months, just more or less.

Reply to Anon

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