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 happened 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 dateby 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
Planning and expectations
Once you know you’re pregnant and you’ve calculated your due date, then what? What are the first things you need to do to get ready to meet your little one? Keeping yourself and your baby healthy is a priority right from the start.
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 macronutrients, 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 limit other foods, like:
- 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. Some examples include:
- Walking
- Yoga
- Swimming
- Bike riding
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 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.
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.