How we calculate your due date
Your pregnancy due date is the calculation of when you can expect to give birth. Our Pregnancy Due Date Calculator can help you make your best guess as to when your little one will enter the world.
You can use a variety of methods to calculate your due date: the first day of your last menstrual period, your conception date (if you know it), your first ultrasound test, or the timing of your IVF transfer.
Last menstrual period method
Knowing the exact date of conception is challenging. Pregnancy can occur if intercourse happens up to five days before ovulation, because sperm can live in your body for those five days, and an egg can be fertilized up to 24 hours after it’s released from your ovary.
If you know the first day of your last period, you can calculate your estimated due date by adding 280 days (or 40 weeks) to that date. This method uses a regular cycle (28 days) and assumes ovulation is around day 14. Accuracy is affected if your cycle is longer or shorter.
Conception date method
Sometimes you know the exact date of conception because you've tracked your ovulation or used an ovulation test. Then you can calculate your due date by simply adding 266 days (or 38 weeks) to that date. This can be more accurate than using the start date of your last period.
Ultrasound method
Your first ultrasound can give you a more accurate estimated due date. Your doctor or midwife can do this by measuring your baby from the crown of their head to the bottom of their bum.
And while an ultrasound is important for all pregnancies, it is especially useful in establishing an estimated due date if you’re someone who has irregular periods.
IVF transfer date method
When you conceive through in vitro fertilization (IVF), you can calculate your due date by using your IVF transfer date. (This is the date that the fertilized embryo is placed in your uterus.) Most transfers occur either three or five days after egg retrieval and fertilization.
Here’s a breakdown of what that timing looks like.
Day-3 embryo transfer
- Add 263 days to the transfer date
- The embryo is 3 days old at transfer
- 266 days (full gestation from fertilization) − 3 = 263 days
Day-5 embryo transfer
- Add 261 days to the transfer date
- The embryo is 5 days old at transfer
- 266 − 5 = 261 days
Naegele’s rule
For everyone who loves a good math equation, here’s another way to discover your due date. Franz Naegele was a famous German obstetrician (1778-1851), and he created this formula:
- Figure out the first day of your last period. (Let’s say that’s January 1, 2026.)
- Count back three calendar months from that date. (So that would be October 1, 2025.)
- Add one year and seven days to that date. (October 1, 2025 + one year + 7 = October 8, 2026.)
- Due date calculated! (October 8, 2026.)
How to tell how far along you are
All mothers-to-be want to know how far they are into their pregnancy. A slightly different calculation than their expected due date, identifying the number of weeks (or even days) you have been pregnant gives you a sense of what your body might look and feel like along the way.
Your pregnancy is measured in gestational weeks beginning with the first day of your last period instead of the date of conception. This official standard means you are considered two weeks pregnant before you conceive!
Your counting proceeds from there. Four weeks after your last period, you hit the one-month mark, eight weeks after it, you’re two months pregnant, and so on.
Note: This can get confusing since not every month is exactly four weeks, which is why medical professionals track pregnancy progress in weeks instead of months.
This is why pregnancies are explained as lasting around 40 weeks, even though your baby takes approximately 38 weeks to grow inside of you. A pregnancy reaches term at 37 weeks, but full term is 39 weeks through 40 weeks and 6 days.
What does this look like if you’re counting days? Well, the first day of your last period is considered day 1 of your pregnancy, and the counting continues from there until you reach 280 days (40 weeks).
Determining your conception date
You know you can use our pregnancy due date calculator to determine more or less when you will get to welcome your baby into the world, but what about figuring out when you conceived your little one? Is that possible if you haven’t been tracking ovulation?
First, we need to define conception date. This is the exact day sperm meets an egg to fertilize it. But this incredible moment in time is nearly impossible to get exactly right. Too many variables exist.
Women ovulate on different days of their menstrual cycle, an egg can live for 24 hours unfertilized, and sperm can survive for a whopping five days in the reproductive tract!
While you may not be able to determine your exact conception date, you can estimate it using these assumptions: your menstrual cycle is 28 days long, you ovulate on day 14, and you conceive on that same day.
If you make all of those assumptions, your conception date is about 38 weeks before your due date. So, if your due date is on January 1, 2027, your estimated date of conception is April 10, 2026. (Just remember that most women’s cycles fluctuate, so these aren’t exact numbers.)
Choosing when to become pregnant
What about a way to calculate when you want to become pregnant? You might have a major life event coming up (like a move, wedding, trip, or medical procedure), and you don’t want to be nine months pregnant when it’s happening. How do you do that math?
If you’re trying to plan a pregnancy, a reverse due date calculator is a useful tool. Once you enter your desired due date, you can figure out when you need to conceive, when your last menstrual cycle should be, and your fertility window for that cycle.
How to use a reverse due date calculator
Choose an ideal due date.
Pick a date that works for you. Don’t forget that babies have minds of their own from the get-go, so buffer your desired date with at least two weeks on either side.
Calculate backward to a likely conception date.
Subtract 266 days (38 weeks) from that ideal due date to arrive at a conception date. This is the day that you are most likely to be ovulating.
Identify your fertility window.
Women are generally fertile for six days in each cycle: the day of ovulation and the five days preceding it. Make note of that date span.
Be aware of your target menstrual cycle.
Your last menstrual cycle should occur two weeks before the conception date you calculated. Being on the lookout for this period will let you know the ideal days to try to conceive.
Planning and expectations
Once you know you’re pregnant and you’ve calculated your due date, then what? What can you expect, and what should you do?
Understand Trimesters.
A pregnancy is divided into three stages called trimesters, which are about three months each in length. Each trimester brings incredible developments to your baby’s growth, and they bring new changes to your body, too.
First trimester (First day of last menstrual period to about 13 weeks)
- Missed period
- Tender breasts
- Possible nausea and vomiting (morning sickness)
- Fatigue
- Needing to urinate more than usual
Second trimester (14 to about 27 weeks)
- Morning sickness often improves
- Energy returns
- Weight gain
- Loosening of joints and ligaments
- If you have gestational diabetes, it will usually be detected during weeks 24-28
- You begin to feel your baby move
Third trimester (28 weeks to delivery / around 40 weeks)
- Back to feeling tired
- More weight gain
- Baby drops into your pelvis (later in the trimester)
- Back pain and breathlessness, often caused by your uterus pressing on your stomach and diaphragm
- Heartburn
These changes are completely normal, and while some may be uncomfortable, they’re all signs that your body is doing what it needs to do to nurture your baby as they grow. And you can do things to alleviate some of your discomfort, such as:
- Eating small, frequent meals to minimize nausea
- Staying hydrated and getting plenty of fiber
- Taking naps
- Yoga, stretching, or sleeping on your side with a pillow under your belly to relieve joint and ligament pain
- Asking people for help (with chores, for example) so you can rest
As your body is changing and your baby is growing, keeping yourself and your baby healthy is the priority.
Schedule a prenatal appointment.
Even if you know you’re pregnant (maybe you’ve taken a pregnancy test or maybe you’re already feeling some morning sickness), it’s important to make an appointment with your doctor or midwife right away.
They’ll confirm your pregnancy and begin the process of checking on your baby’s health and growth, as well as your own. Continuing these prenatal appointments throughout your pregnancy is critical.
Focus on your health.
Start taking prenatal vitamins and make sure you’re getting enough of critical nutrients, such as:
- Folic Acid: 400-600mcg daily
- Iron: 27mg daily
- Calcium: 1000mg daily
- DHA: 200mg - 300mg daily
- Protein: 75-100g daily
Be sure to check with your doctor about how much of the above you need. Amounts may vary based on your pregnancy and medical history.
You’ll also want to eat a variety of vitamin-rich, healthy foods, like:
- Leafy greens such as spinach and kale
- Lean proteins such as chicken, fish, and legumes
- Whole grains such as quinoa and brown rice
- Dairy products such as milk, yogurt, and cheese
- Colorful fruits and vegetables such as carrots, oranges, and blueberries
And finally, you’ll want to avoid or limit other foods, including:
- High-mercury fish like shark and swordfish
- Raw and undercooked meats
- Unpasteurized dairy products
- Excessive caffeine (limit to 200mg a day, or 2 small cups of coffee)
- Alcohol (avoid completely)
Diet is not the only factor to consider while you’re pregnant. Exercise provides all kinds of benefits to you and your baby.
According to the U.S. Centers for Disease Control and Prevention (CDC), pregnant women should aim to get at least 150 minutes of exercise per week, which breaks down to half an hour, five days a week. Exercise has been shown to reduce:
- The risk of excessive weight gain
- The risk of gestational diabetes
- Postpartum depression
- Symptoms of discomfort (like back pain)
What kind of exercise should you do? Whatever feels good, is familiar, and is something you like (and is approved by your healthcare provider). Some examples include:
- Walking
- Yoga
- Swimming
Be flexible with your due date calculations.
Keep in mind that due date calculations are estimates. Only about 5% of babies are born on their exact due date. Most babies are born within two weeks before or after your estimated due date. Early pregnancy ultrasounds tend to be the most accurate dating method.
Your healthcare provider may adjust your due date based on ultrasound measurements, particularly if there's a significant difference between different dating methods. Early ultrasounds are usually given priority in determining the final due date.
Frequently asked questions
Which calculation method is most accurate?
The most accurate method of calculating a due date is ultrasound dating, especially in the first trimester (6-12 weeks). Early ultrasounds can predict the due date within a 3-5 day range.
Ultrasound dating is especially useful if you have irregular periods or are unsure of your last menstrual period.
What if I have irregular periods?
The last menstrual period method may not be as accurate if you have irregular menstrual cycles. In this case, you may get more reliable results by using your conception date (if you know it) or doing an early ultrasound.
It’s always recommended to consult with your healthcare provider for the best approach.
What if I don’t know the date of my last period?
If you’re unsure about when you had your last period, an ultrasound can be used to determine your due date. A “dating” ultrasound measures your baby’s length to establish a pregnancy timeline.
What week counts as full term versus early term?
According to the American College of Obstetricians and Gynecologists and the Society for Maternal Fetal Medicine, a full-term pregnancy is from 39 weeks through 40 weeks and six days.
A pregnancy is considered early term from 37 weeks through 38 weeks and six days, while a late term pregnancy is from 41 weeks through 41 weeks and six days.
Waiting to deliver your baby until at least 39 weeks gives your baby’s lungs, liver, and brain the critical time they need to grow and gives your baby the best chance for a healthy start.
When will I go into labor?
According to the American College of Obstetrics and Gynecologists (ACOG), the average length of a pregnancy is 280 days (40 weeks). Although you can’t know exactly when you’ll go into labor, most women give birth between 38 and 41 weeks.
What should I talk about with my health care team before my due date?
Ask your OB-GYN or midwife when they want you to call them.
Do they want to talk to you the minute you suspect you’re in labor? Do they want to talk with you once you’ve passed full term and are heading into late term? You should also ask them where they can be reached after hours.
That said, don’t be afraid to call your healthcare provider any time you have a question or need reassurance.
Can my due date change?
Yes, your due date can change as more information is gathered about your baby, specifically data like ultrasound measurements. These can give your obstetrician or midwife a more detailed understanding of your baby’s measurements, helping determine a more accurate due date.
How can a pregnancy due date calculator help me if I already know my due date?
In that case, you can use the pregnancy due date calendar to better understand the timeline of your pregnancy. You can see when specific milestones will most likely occur, and you can schedule prenatal tests and visits.
What should I do if I go past my due date?
Once you reach 41 weeks, your health care team will probably do some tests on you and the baby. A non-stress test and biophysical profile (an ultrasound) are typical.
If those tests show that your baby is healthy and your levels of amniotic fluid are normal, your doctor or midwife may want to wait and see if you can go into labor on your own. But if they worry your baby may not be doing well, you may decide together that you should be induced.