How to figure out the weeks, months, and trimesters of pregnancy

Pregnancy can sometimes feel like a lot to figure out, especially if it’s your first time. After you’ve gotten that big fat positive on a pregnancy test, figuring out exactly how pregnant you are at any given point can be especially tricky. Because, although pregnancy is measured in weeks from a medical point of view, most other people seem to talk about pregnancy in months. 

Tracking pregnancy is important to help you stay on top of your baby’s development and to measure their growth. “The [baby] and pregnancy as a whole have known milestones that are reached and change week by week,” says obstetrician and gynecologist Dr. Charlsie Celestine from New Jersey, US. “Talking about pregnancy by month is just too broad for a medical provider.” 

A helpful tool like the Flo pregnancy tracking app can help you to follow the progress of your pregnancy outside of your scans and appointments. It offers week-by-week updates based on your personal experience and helps you to learn exactly what’s going on inside your changing body. 

Even if you track your pregnancy routinely, you might still find yourself unsure of the answer when someone coos at your bump and asks you how many months you are. So, how do you figure out how to convert pregnancy weeks to months? Keep reading to find our pregnancy weeks to months chart and learn about all the possible ways a pregnancy can be tracked. 

Key takeaways

• Pregnancy lasts approximately 40 weeks, counted from the first day of your last menstrual period (LMP).
• Medical professionals prefer to count pregnancy in weeks, rather than months.
• There are 3 trimesters in pregnancy, each with their own developmental milestones.
• Understanding your pregnancy in weeks, months, and trimesters helps you track your baby’s growth and how your body is changing, as well as preparing you for birth. Flo's pregnancy tracking app can help you with this.

How many weeks of pregnancy are there?

Pregnancy typically lasts about 40 weeks (or 280 days). But every pregnancy is different, and some babies may arrive preterm (before 37 weeks), while others may not be born until up to two weeks after their due date (late term). Pregnancy is counted from the first day of your last menstrual period, which we’ll explain more about below. This means an extra two weeks are counted at the beginning of your pregnancy when you aren’t actually pregnant.

Check out our online pregnancy week-by-week guide for what to expect at every stage or download the Flo app for pregnancy tracking that’s tailored to you. 

Figuring out your week of pregnancy

To determine your current week of pregnancy, you need to find out your due date, and your doctor will be able to help you do this using the date of your last menstrual period. It will later be confirmed by a scan at the end of your 1st trimester, often called a dating scan.

When you know your due date, you can count how many weeks you are away from that date and then deduct that number from 40 (the total number of weeks in a standard pregnancy). If that sounds complicated, a pregnancy tracking app like Flo can figure out which week of pregnancy you’re in based on your due date.  

If you want to try estimating when your baby may be due before you see a doctor or have a scan, start by identifying the first day of your LMP. “This is the first day you started bleeding,” Dr. Celestine explains. Then — and you may want to grab a pen and some paper for this — “Count back three calendar months from that date. Add one year and seven days to that date, and that’s your due date, which is the date you turn 40 weeks pregnant if you have a 28-day cycle.”

For example, if your LMP began on January 1, you would be considered four weeks pregnant on January 29. Three months earlier than your LMP is October 1. So, after adding seven days, your due date would be October 8. Our online due date calculator can help you if you’re not a fan of math. 

If you’re unsure of your LMP or have irregular cycles, an early ultrasound will give a more accurate measurement of your pregnancy’s progress. It’s worth remembering, though, that your due date is just a rough guide. Only about 5% of babies are born on their estimated due date.

How many months of pregnancy are there?

While pregnancy is often described as lasting 9 months, it’s not that clear-cut. We know that a full-term pregnancy lasts around 40 weeks and that each month has roughly four weeks, but because calendar months vary in length, translating weeks into months isn’t straightforward. 

Check out the chart below for a general guide on how to convert pregnancy weeks into months and trimesters, or give our pregnancy weeks to months calculator a try.

Pregnancy weeks to months chart

Figuring out your month of pregnancy

So if friends or family ask how many months pregnant you are, you can use the chart above to convert your weeks of pregnancy into months. But remember that this isn’t an exact science because doctors never count pregnancy in months, and some months are longer or shorter than others. 

Instead, you could use the Flo pregnancy tracking app to follow your progress and receive daily updates to help you understand your baby’s development.

How many trimesters of pregnancy are there?

Pregnancy is divided into 3 trimesters, each lasting about 13 weeks. Figuring out your trimester of pregnancy can help you prepare for prenatal appointments and check off key milestones. Dr. Celestine breaks them down:

• 1st trimester: from conception to 13 weeks. “Major organs begin to form during this stage,” she says.
• 2nd trimester: from 14 weeks to 27 weeks. “Eyes and ears begin to move into their proper position on the face. Fetal movement starts and can be felt,” she explains.
• 3rd trimester: from 28 weeks onward. “The brain and lungs fully develop,” notes Dr. Celestine.

Understanding your pregnancy by week, month, and trimester

Pregnancy is a fascinating journey, and each week brings new milestones for you and your baby. Understanding your timeline in weeks, months, and trimesters can make the experience less overwhelming and help you prepare for what’s to come. 

The Flo pregnancy tracking app offers personalized updates, helping you understand your baby’s development and your body’s changes throughout your journey. “So much changes within a few days as pregnancy progresses,” explains Dr. Celestine. Just remember to consult your doctor if you have any questions or concerns about your pregnancy timeline.