How to Calculate Months of Pregnancy

As soon as you learn you're pregnant, you'll probably start counting nine months on your fingers to figure out when your baby will be born. But then your health care provider mentions that you're likely to go into labor around 40 weeks, and things don't seem to line up, because 40 weeks sounds more like 10 months.

Your gestational period is, in fact, 40 weeks long, but the counting starts on the first day of your last period (LMP). You aren't technically pregnant during weeks 1 and 2, and by the time you see a positive result on a pregnancy test, you're already considered four weeks pregnant. The first month has already passed!

You can count your gestational period in weeks or months. Your health care provider will use weeks, but you might start using months in conversation when you get further along. Your friends and coworkers might find "I'm 8 months" easier to understand than "I'm 32 weeks."

In this article, we give tips for figuring out how many weeks or months along you are throughout your pregnancy.

Pregnancy Weeks vs. Pregnancy Months

There are about 40 weeks in a full-term pregnancy. If you assume that a month is exactly four weeks long, that makes 10 months of pregnancy. The problem with this calculation is that it assumes that each month lasts 28 days—but most of our calendar months last 30 or 31 days.

To confuse matters even more, gestational age is calculated based on your last menstrual period (LMP) instead of your conception date. By the time you receive a positive pregnancy test, you're usually about 4 weeks pregnant.

The truth is that a full-term pregnancy lasts between nine and 10 months. To establish clarity about your stage of pregnancy and estimated due date, your practitioner will track your pregnancy in weeks rather than months.

You can also estimate your due date with an online calculator, like the one below.

Pregnancy Trimesters by Week

While you might explain your pregnancy in weeks or months, many expectant parents also describe their pregnancy in trimesters. Each trimester lasts about 12 to 13 weeks. There are three trimesters in a full-term pregnancy.

First Trimester

The first trimester usually includes weeks 1 through 13, or months 1 to 3. This is considered a short trimester because many people don't know they're expecting during the early weeks.

Second Trimester

The second trimester usually includes weeks 14 through 27, or months 4 to 6. Many parents consider this to be the easiest and most comfortable trimester.

Third Trimester

Your third trimester may include weeks 28 to 40, or months 7 to 9. But many people go into labor before week 40, and some may stay pregnant a week or two longer.

Calculating Pregnancy Weeks

Your practitioner will count your pregnancy in weeks to accurately assess the health of you and your baby. If you're unclear about your pregnancy week, ask for your provider's guidance.

Once you get your estimated week of pregnancy, you can use that information (as well as your due date) to track your pregnancy at home. Consider using a calendar, an online app, or other tools.

Counting Pregnancy Months

Understanding your pregnancy in weeks, months, and trimesters can be confusing enough. But you'll also be faced with plenty of pregnancy questions from others. When someone asks: how far along are you? It's usually easiest to reply in months.

Here are a few different ways to break down your pregnancy month.

Pregnancy by month: emphasis on first trimester

This monthly breakdown assumes that early pregnancy is very important. Since many people don't know they're pregnant during the first few weeks, month one is extended to six weeks.

• Month 1: Week 1 through week 6
• Month 2: Week 7 through week 11
• Month 3: Week 12 through week 16
• Month 4: Week 17 through week 20
• Month 5: Week 21 through week 24
• Month 6: Week 25 through week 28
• Month 7: Week 29 through week 32
• Month 8: Week 33 through week 36
• Month 9: Week 37+

Pregnancy by month: emphasis on third trimester

This weekly pregnancy breakdown includes the last normal menstrual cycle, and it considers that some people have their babies earlier than forty weeks.

• Month 1: Week 1 through week 4
• Month 2: Week 5 through week 8
• Month 3: Week 9 through week 12
• Month 4: Week 13 through week 16
• Month 5: Week 17 through week 20
• Month 6: Week 21 through week 24
• Month 7: Week 25 through week 28
• Month 8: Week 29 through week 32
• Month 9: Week 33+

Pregnancy by month: emphasis on calendar dates

This breakdown takes the date of your last menstrual period, and it uses that date in subsequent months. For example:

If the date of your last menstrual period was April 17:

• Month 1: April 17 through May 16
• Month 2: May 17 through June 16
• Month 3: June 17 through July 16
• Month 4: July 17 through August 16
• Month 5: August 17 through September 16
• Month 6: September 17 through October 16
• Month 7: October 17 through November 16
• Month 8: November 17 through December 16
• Month 9: December 17+

As you can see from these schedules, there are different ways to describe how many months pregnant you are. There's no right or wrong way to calculate it.