Calculating age of baby

This may be a strange question, but how have mummies and daddies been calculating the age of baby? Until recently we had been announcing the age of baby in months according to every 4 weeks. But we just realised that whilst there's only 12 months in a year, if you count every 4 weeks then there's actually 13 months in a year (ie. 52 weeks as opposed to 48). Do you count per 4 weeks or by the same date each month. Eg, our son was born 18 March so 1 month would be 18 April, 2 months would be 18 May, etc