Many people wonder about pregnancy weeks and months. Especially new moms who are counting down the month’s until their due date. Other reasons to know this may include; weeks or months left of school, number of months until a trip you might be taking, or counting down until another big life event. Because our calendar is based on the “Gregorian Year,” weeks into months isn’t as easy as you would think. We use a rough estimate of four-weeks to every month, but it’s a bit more involved. This article explains how weeks actually calculate into months, and a handy chart to help.

How Many Months is 20 Weeks?

20 weeks is commonly known as, five months. However, the closer average is 4.35 months. This is because some months have 30 days, others have 31, and February sometimes has only 28 days. Because of this, it is more accurate to divide the weeks of the year, rather than count up in weeks. 

This is because calculating the passage of time is relevant to the earth’s orbit around the sun and there are variations depending on the year. The Gregorian year is actually 52.18 weeks long, and there are actually 365.42 days in a year. Every fourth year is a “leap year,” which means that year there is an extra day in February. On leap years there are 366 days, this actually leads up to around 53 weeks during a leap year.

Calculating the Number of Months for 20 Weeks

Now let’s calculate the numbers. If a non-leap year is 365.42 days, you just divide that by 7 (days a week) and you will get 52.18. On a leap year, you divide 366 by 7 and you will get 52.28.

To standardize how many months is 20 weeks, you would need to divide 52.18 by 12 months. The sum of this number is 4.35 weeks in a month. Then you divide 20 weeks, by 4.35. For example, 20 ÷ 4.35 = 4.59 months. So, 20 weeks isn’t quite 5 months. With this calculation, it takes about 23 weeks to arrive at 5 full months.

During a leap year, divide 52.28 by 12 and this gives you 4.36. Divide 20 weeks by 4.36 and you get roughly 4.6 months. It is a little bit closer to 5 months, but only by a day. The good news is on a leap year, 23 weeks is precisely 5 months! And people with a February 29th birthday get to celebrate!

For calculating how many months is 20 weeks in a pregnancy, a chart by trimester of pregnancy will help you see where you are.

Weeks To Months Reference Chart 

The first month of pregnancy is actually month zero. This is because the first two weeks of this month you weren’t pregnant. This month begins the the first day of your last period. Many women don’t ovulate on day 14 of their cycle, so you can become pregnant any day after your period ends. A small number of women may even have delayed ovulation and conception may have occurred more towards the last week or week 4. One month of pregnancy is calculated a four full weeks after the first day of the last period.

This chart shows in weeks the approximate month of pregnancy you are in. At the end of the 20th week, you are just finishing the fourth month of pregnancy with another week until you are in your fifth month. (See the highlighted row)