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Is pregnancy 9 or 10 months long?

Updated: Mar 8, 2020

This is a popular pregnancy debate across many online forums and within the pregnancy community. Most people know that pregnancy consists of three trimesters – the first, second and third trimester. However, many women get confused as to how long pregnancy actually lasts for.

The definition of a trimester is ‘a period of three months, especially as a division of the duration of pregnancy.’

If this is the case, and pregnancy is classed as three, three monthly trimesters, why do some women class pregnancy as ten months long?

A full-term pregnancy is classed as 40 weeks, and many women count four weeks as a month while they are pregnant, making the result a ten-month pregnancy. This is a common misconception by many people that each month contains only 4 weeks. On average there are actually 4.33 weeks in a month, so classing pregnancy as ten months long is in fact inaccurate.

Pregnancy is calculated from the first day of your last missed period. If your last missed period is on January 1st, then you are one month pregnant on February 1st, not 4 weeks after the first day of your last period.

Your pregnancy dates will be calculated from the first day of your last period. However, note that your menstrual period and ovulation are counted as the first two weeks of pregnancy, therefore actual gestation in weeks (from the minute the sperm meets the egg) is approximately 38 weeks long, depending on the length of your cycles. Although technically classed as pregnant, you are not actually physically pregnant for the first 2 weeks.

The most common way to figure out your due date, including the way that doctors and midwives will calculate it is by adding 280 days (40 weeks) to the first day of your last menstrual period (assuming a 28 day cycle). This is approximately 9 months and 1 week.

If you give birth on your due date at 40 weeks pregnant, your baby’s gestational age is 38 weeks and you will have had a 9 month long pregnancy. A ten month pregnancy would in fact have lasted for 43 weeks.

So there you go, no matter how tough it feels, pregnancy really is only 9 months!

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