Pregnancy dates can get confusing. You may be told you are a certain number of weeks into your pregnancy, a certain number of months, or you may be told you are in a certain trimester.
Then you have a due date, which can sometimes change as your pregnancy progresses.
So, what is a trimester? And how many weeks are in a pregnancy month? Read on to find out about weeks, months and trimesters, and how to calculate your due date.
How many weeks in a typical pregnancy?
A typical pregnancy lasts 40 weeks. That’s 280 days and is calculated from the first day of your last menstrual period (rather than the day you conceived).
Only a small number of pregnancies last 40 weeks, with most babies being born a week or so before or after. Anywhere between 39 and 41 weeks is considered full term.
Babies born before 38 weeks and 6 days are referred to as early term, and those born before 36 weeks and 6 days are considered premature.
Babies born after 41 weeks are late term, and those born after 42 weeks are post term.
How many weeks are in a pregnancy month?
This is where it gets a little confusing. The 40 weeks you are pregnant are counted as 9 months. But, you may be asking why it’s not counted as 10 months, as there are 4 weeks in a month (40 weeks divided by 4 weeks equals 10 months, no?).
Well, while 4 weeks is 28 days, most months are longer, with around 30 or 31 days. Therefore, each month is approximately 4.3 weeks.
What is a trimester?
Your pregnancy is divided into trimesters. Each trimester lasts either 13 or 14 weeks (40 weeks doesn’t divide evenly into 3, hence the breakdown isn’t exact).
Your first trimester lasts from week 1 to week 13 (13 weeks), the second from week 14 to week 27 (14 weeks), and the third from week 28 to week 40 (13 weeks).
How can I calculate my due date?
To calculate your due date, you will need to know the date of your last menstrual period. You then add 40 weeks to this date.
Due dates are calculated from the date of your last period rather than the date of conception, as this is more reliable. Sperm can survive inside the body for up to 5 days and an egg can be fertilized up to 24 hours after it’s released, meaning knowing exactly when you conceived can be tricky.
So, by the time you take a pregnancy test on the day your period is due, you are already 4 weeks pregnant.