Calculating your pregnancy weeks and trimesters

Calculating how many weeks pregnant you are and working out exactly when the trimesters change can be confusing especially as technically for the first 2 weeks of this 40 weeks you are not even pregnant yet!

Number of weeks in a pregnancy

It is generally understood that a pregnancy is 40 weeks long. Your due date is calculated as 40 weeks (or 280 days) from the first day of your last period. If you have a regular 28 day cycle this can be very accurate however if you have irregular periods the date can be more difficult to pinpoint. Often a ‘dating scan’ is used to determine the baby’s gestation.

Number of months in a pregnancy

9 months is generally the duration of a pregnancy but 9 months doesn’t divide equally into 40 weeks so in reality a pregnancy month is approximately 4.3 months.

Number of weeks in a trimester

Again, the 40 weeks don’t go neatly into the three trimesters. It is generally accepted that the trimesters can be dividied as follows:

First trimester (Weeks 1 to 13)

  • Month 1 covers weeks 1 to 4
  • Month 2 covers weeks 5 to 8
  • Month 3 covers weeks 9 to 13

Second trimester (Weeks 13 to 27)

  • Month 4 covers weeks 14 to 17
  • Month 5 covers weeks 18 to 22
  • Month 6 covers weeks 33 to 27

Third trimester (Weeks 28 to 40)

  • Month 7 covers weeks 28 to 31
  • Month 8 covers weeks 32 to 35
  • Month 9 covers weeks 36 to 40

So what week am I ?

The terminology gets very confusing here!

The best way to understand this is to think about your birthday. When you are under one year old you are never called one – not until you actually celebrate your birthday. It is the same with weeks pregnant. You are not actually, for example, 10 weeks pregnant technically until you reach week 11 you can be called 10 weeks or 11 weeks! So, at the other end of the pregnancy, you are really only 40 weeks at the end of the 40th week, not the beginning. So don’t get your due date and 40 weeks confused!

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