How long is pregnancy? The weeks, months, and trimesters in full-term pregnancy explained

The first few months of pregnancy can be a really exciting time, and one piece of information that can feel particularly crucial is your due date. Pop culture has long portrayed pregnancy as a nine-month countdown to birth. But how long is pregnancy? Working this out can actually be a little bit more complicated than that. 

“From a medical standpoint, doctors always talk in weeks and days,” says obstetrician, gynecologist (OB-GYN), and Flo medical board member Dr. Charlsie Celestine. “A full-term pregnancy is 40 weeks long, which equals 10 months. Yet commonly, people talk about pregnancy as being nine months long.” 

Hang on. 10 months, not nine? How can that be the case? According to the National Health Service in the United Kingdom, the average length of human gestation is 280 days or 40 weeks, and that time starts from the first day of your last period, when you’re not technically pregnant yet. That’s because ovulation (which happens midway through your cycle) and fertilization haven’t occurred yet — but more on that later.

No two pregnancies are the same, and pregnancies can vary in length. Here, Dr. Celestine explains why pregnancy is often referred to as nine months and how your due date is calculated.

How long is a full-term pregnancy in months and weeks?

If most full-term pregnancies (when a baby is born between 39 weeks and 40 weeks and 6 days) last 10 months, then you’ll probably be wondering where the idea of nine months comes from. 

This confusion about how we refer to pregnancy length could be related to the fact that most people don’t find out that they’re pregnant until they’ve missed a period, which is already approximately four weeks into the pregnancy. “Everything is based on your cycle,” says Dr. Celestine. 

“Ovulation [when an egg is released from your ovaries ready for fertilization] typically takes place around two weeks after the first day of your last period, and this is when a baby would be conceived. But because you won’t know that you’re pregnant until you’ve missed your next period, approximately two weeks later, you’ll already be four weeks pregnant when you get your positive test result. At that point, you’ll have approximately 36 more weeks to go,” Dr. Celestine says.

And 36 weeks equates to nine calendar months, which is why many people commonly refer to pregnancy as being nine months long. Makes sense, right?

How is a due date calculated and how do I know my current week of pregnancy?

If you suspect that you’re pregnant or have taken an at-home pregnancy test that’s come back positive, the best next step is to reach out to your health care provider. During your first appointment, they will be able to confirm that you’re pregnant and work out your estimated due date. 

Most health care providers use Naegele's rule to calculate the baby’s due date. This rule was invented by the obstetrician Franz Naegele in the 19th century, and it’s still being used today. It means your health care provider will work out your due date like this: 

• Firstly, your health care provider will determine the first day of your last period. 
• Secondly, they will then count back three calendar months from that date. 
• Thirdly, they will add one year and seven days to that date. Rather than working this out with a pen and paper, your health care provider will use a due date calculator to estimate your date.

Knowing your due date can feel like a milestone moment in your pregnancy — but don’t worry, you aren’t expected to figure it out yourself. If you track your periods with an app like Flo, then you’ll be able to tell your medical professional when the first day of your last period was. They will then be able to do the calculation for you. You can also use a due date calculator. 

Does an irregular cycle affect your due date?

Tracking your period can be a little bit more difficult if you have an irregular cycle. However, if this is something you experience, don’t worry. An irregular cycle won’t affect the length of your pregnancy. “A lot of people’s cycles are irregular, so sometimes we can’t use your period as a predictor. We have to measure the fetus on an ultrasound instead,” says Dr. Celestine. 

During your first trimester (the first 14 weeks of pregnancy), how far along you are (gestational age of pregnancy) is determined by your baby’s crown-rump length. “This is literally the length of the fetus as seen on the ultrasound,” explains Dr. Celestine. Your health care provider will note how long your baby is in centimeters from the top of their head (crown) to the bottom of their buttocks (rump). 

“This gives us the estimated gestational age,” adds Dr. Celestine. Your health care provider can take these measurements between weeks six and seven of pregnancy up until week 14. After week 14, your gestational age is determined by other methods (including measuring your baby’s head circumference, abdomen, and thigh bone). This is called biometry. 

Your health care provider may compare your estimated due date based on your last period with any information from your first ultrasound. In the first few weeks after you’ve found out you’re pregnant, you may be given a couple of estimated due dates as your health care provider tries to establish the most accurate estimate. Make sure that both you and your health care provider are using the same due date to keep track of your pregnancy.

It’s important to remember that whether you’re working from the first day of your last period or the crown-rump length on an ultrasound, a due date is just a rough estimate of when the baby will come. “It can be off by a few days, either way,” says Dr. Celestine. Most babies are born between 38 and 41 weeks of pregnancy. In one study of nearly 19,000 women in Australia, just 5% of births actually happened on their due dates. The due date is more of a guide to track how far along you are in your pregnancy than a set-in-stone deadline of when your baby will arrive.

How long is a trimester, and why do we use them to measure a pregnancy?

Most of us know that pregnancy is measured in three trimesters. Different symptoms and milestones are associated with each trimester. However, just like the length of a pregnancy, there has been some confusion about how long each trimester lasts. 

“People used to think trimesters are 12 weeks each, but they’re not — they’re around 14,” explains Dr. Celestine. “The first trimester goes to 13 weeks and 6 days. The second trimester is from 14 weeks to 27 weeks and 6 days. And the third trimester goes from 28 weeks to 42 weeks.” 

As mentioned above, a full-term pregnancy is one that is 39 to 40 weeks long. If you’re still pregnant at 41 to 42 weeks, your health care provider will usually recommend an induction of labor. 

Inducing labor is a medical process that helps your body get ready for and go into labor. A technique that is sometimes offered to help encourage labor is a membrane sweep.

Knowing which trimester you’re in can give your medical professionals a framework for monitoring benchmarks in your baby’s development and changes your body is experiencing. 

“Certain things happen at certain weeks,” says Dr. Celestine. “The baby will be hitting certain marks in terms of growth. Getting it relatively right is important. If something seems a little bit off, we give it a bit of time to recheck it again in order to confirm if it’s a true issue or not.” 

If you’re at all confused or concerned about what your health care provider is looking for at different points in your pregnancy, then reach out and ask questions. It’s so crucial that you feel supported and understand what is happening in your body and with your baby. 

Are all pregnancies the same length?

It’s important to remember that your estimated due date is just a rough guide, because not all pregnancies are the same length. As explained above, most pregnancies are around 40 weeks long, but a baby can safely arrive at various points within this time frame. There are a number of common terms that your health care provider may use to describe when your baby may arrive. They include:

Premature

 A premature or preterm baby is a baby that’s born before 37 weeks. “So that’s up to 36 weeks and 6 days,” says Dr. Celestine. According to the Centers for Disease Control and Prevention (CDC), in 2020, 10% of babies born in the United States were born preterm, so roughly 1 in 10. 

If your baby is born preterm, they may need support from the neonatal or newborn intensive care unit (NICU) at your hospital. This could include help with breathing, regulating temperature, and feeding. 

Early term

An early term baby is one that’s born between 37 weeks and 38 weeks and 6 days. “A baby born at 37 weeks might not have to be in the NICU and should be able to breathe on its own,” says Dr. Celestine. “But it’s still what we call early term.” Some babies born at this gestational age do have temporary breathing issues or may need extra help with breastfeeding, maintaining temperature, and managing sugar levels.

Full term

A full-term baby is one that is born between 39 weeks, (the week before your due date), and 40 weeks and 6 days. The CDC notes that 57% of all babies born in the United States in 2020 were full-term births. “If we’re scheduling a cesarean section, we try to do it in the 39th week,” explains Dr. Celestine. “This is because we try to do it before the woman goes into labor naturally.”

Late term

A late-term baby is born from week 41 up to week 41 and 6 days. In the United States, most OB-GYNs offer and recommend inducing labor between 41 weeks and 41 weeks and 6 days. This is because at this point, the risks of continuing the pregnancy increase. Dr. Celestine says, “We normally induce labor if you get to 41 weeks or beyond because at that point, the baby is safer out than in.” However, it’s important to remember that every pregnancy is different, and your health care provider may recommend inducing labor before 41 weeks if they think it’ll be better for you and your baby’s health.

Post-term

A post-term baby is one born at week 42 and beyond. The most common reason for a baby to be born post-term is because the initial estimated due date was calculated incorrectly. 

What is the average pregnancy length for first-time parents?

While your due date may help you get a better idea of how far along in pregnancy you are, unless you’re having a planned C-section, it’s impossible to know exactly when your baby will be born. 

“Most people deliver between 39 and 41 weeks,” says Dr. Celestine. “But statistically, first pregnancies are likely to be a little longer.” Research into how long pregnancy is has varied. One analysis highlighted that 15% of first babies are likely to arrive after 40 weeks, compared to 10% of subsequent babies. However, the same study noted that 12% of first babies were likely to be born before 37 weeks compared to 10% of other babies. 

So, it’s worth remembering that your due date may give you a general idea of when your baby will be born, but there’s no way to know for sure until the big day actually arrives.

What can affect the length of pregnancy?

If you’ve had a baby before, you may wonder if your second pregnancy and birth will be similar to your first. One of the biggest factors in having a preterm birth is if you’ve had one before. If you gave birth before 37 weeks in a previous pregnancy, talk to your health care provider about extra monitoring or support to help lower your risk of having another preterm birth. 

If you’ve had a baby who made a late arrival in the past, then this can also increase the chances that your current pregnancy will go to full term or beyond. 

However, it’s important to remember that every pregnancy is different. “Your first pregnancy doesn’t always dictate what will happen next time. If you have a change in a partner or a change of sperm donor, that can make a difference to gestation time,” says Dr. Celestine. One study notes that you could be at an increased risk of early birth or your baby having a lower birth weight if you have babies with two different partners. However, Dr. Celestine says, “Mainly, just as everybody is unique, every baby and pregnancy is unique.”

One thing that will carry over from pregnancy to pregnancy, though, is risk factors for preterm birth. “If a pregnant person has a condition like high blood pressure or diabetes, or advanced maternal age (if you’re over the age of 35), then we’ll need to take this into account across all their pregnancies,” says Dr. Celestine. 

How long is pregnancy: The takeaway

Pregnancy is closer to 10 months in length. And while your due date is a great point of focus, it’s important to remember that only about 5% of people actually give birth on their due date. 

Your due date is, of course, a useful guide for roughly when you may meet your baby. However, unless you’re having a planned C-section or induction, it’s impossible to know for sure when your baby will be born. If you’re feeling worried or confused about how long pregnancy is or about your due date, then speak to your pregnancy health care provider. They’ll be able to outline which milestones your baby is hitting in their development, what to expect at certain points in your pregnancy, and as the time draws closer, when you can expect to give birth.