Your Estimated Due Date
When you’re pregnant, your due date is the estimated date when you’re expected to deliver your baby. Keep in mind that due dates are always estimates. Every pregnancy is different, and few babies are born on their exact due dates.
During your first prenatal exam, your health care provider will ask questions to try to predict your due date as precisely as possible.
Even though it’s not 100% accurate, knowing your due date helps your provider track your baby’s growth more accurately. It allows them to schedule various prenatal tests at the right times. And it helps them make better decisions about labor and delivery, including preterm labor.
How to Calculate Due Date
There are many ways to calculate your due date. The method you and your doctor use depends on what information is available, such as the date of your last period, the date of conception or IVF transfer, or an ultrasound scan done in your first trimester.
Due date based on last menstrual period (LMP)
Most pregnancies last 40 weeks (or 280 days) from the first day of your last menstrual period (LMP). This method of estimating due date is known as “Naegele’s rule,” named after the German obstetrician Franz Naegele, who first proposed it.
There are two ways to calculate your due date using your LMP:
- Add 40 weeks (or 280 days) to the first day of your last period.
- Or use the shortcut: Subtract three months from the first day of your LMP, then add one year and seven days.
This method assumes that you have a regular 28-day menstrual cycle. If your cycle is longer or shorter, your doctor may adjust your due date accordingly.
Due date according to conception
A full-term pregnancy typically lasts about 38 weeks (266 days) from the date of conception. If you know exactly when you conceived, you can estimate your due date by counting 38 weeks forward from that day.
Due date using ultrasound
An ultrasound scan done during the first trimester of pregnancy (which lasts 13 weeks and six days from the first day of your LMP) is considered the most accurate way of estimating your baby’s age and your due date.
Your doctor will measure the crown-rump length (CRL). That’s the distance from the top of your baby’s head (the crown) to its bottom (the rump). By comparing your baby’s CRL to standard growth charts, they can estimate how far along your pregnancy is and calculate your due date.
Typically, a due date calculated using a first-trimester ultrasound is only off by five to seven days, says Anushka Chelliah, MD, a maternal-fetal medicine specialist at Pediatrix Medical Group in Houston. If it differs from your LMP-due date by more than seven days, the ultrasound due date gets priority, she says.
You can still get an ultrasound after the first trimester, but second- or third-trimester ultrasounds give less precise due dates than first-trimester scans.
“The earlier that you get an ultrasound, the closer (the estimated due date) will be to true,” says Loralei Thornburg, MD, a professor of obstetrics and gynecology at the University of Rochester Medical Center in Rochester, NY.
Pregnancy wheel calculator
A pregnancy wheel is a simple tool made of plastic or cardboard that helps you calculate your due date. To use one, you usually align an arrow on the tool with the date of your last menstrual period. The wheel will then display your estimated due date. Pregnancy wheels and other types of due date calculators are also available online or as apps.
If you conceived through in vitro fertilization (IVF), you can calculate your due date based on the date of your embryo transfer. Embryo transfers are usually done three or five days after egg retrieval and fertilization. For a day-three embryo transfer, your due date is 263 days from the transfer date. For a day-five transfer, it’s 261 days from the transfer date.
How Accurate Is a Pregnancy Due Date?
The most accurate way to estimate the due date is with an ultrasound done during the first trimester. But even that tells your doctor more about how far along the pregnancy is than when exactly the baby will arrive.
Only about 5% of babies are born on their estimated due dates, and just 35% are born during the same week of that estimated date.
Factors Affecting Due Date
Some babies arrive earlier than expected, while others take a little longer.
Delivery after the estimated due date is linked to genetics. If others in your immediate family have had overdue pregnancies, or if you’ve had a late delivery before, there’s a greater chance your current pregnancy could go past the due date as well.
Several things can raise the chances that a baby will arrive before the estimated due date. These include:
- Previous preterm delivery
- A multiple pregnancy, such as twins or triplets
- Pregnancy through IVF
- A short cervix or one that shortens during the second trimester
- Health conditions such as high blood pressure, diabetes, infections (UTIs, STIs, bacterial vaginosis), vaginal bleeding, or placenta previa
- Certain developmental conditions in the baby
- Being underweight or having obesity before pregnancy
- Closely spaced pregnancies (getting pregnant less than six months after giving birth)
- Previous C-section or uterine fibroid removal
- Being under 18 or over 35
- Being Black or American Indian/Alaska Native
- Having no health care during pregnancy or starting it late
- Smoking, alcohol, or drug use
- Domestic violence
- Lack of social support
- Stress
- Spending long hours on your feet
- Exposure to harmful substances in the environment
During regular prenatal checkups, your doctor will watch out for any signs that you might deliver before the estimated due date. These checkups are also a good time to learn about the symptoms of preterm labor and what steps to take if they appear.
Because babies don’t always arrive on schedule, it’s a good idea to be ready a few weeks early — ideally by week 36. This includes finalizing your birth plan, packing your hospital bag, and making any other arrangements you need to help things go smoothly when you go into labor.
Takeaways
Your pregnancy due date is an estimate of when you’re likely to deliver. It’s calculated using the most reliable information available, such as the date of your last period, date of conception, IVF transfer date, or a first-trimester ultrasound scan. But only a small percentage of babies are born exactly on their due date, and many things can affect the timing of your delivery. Get regular prenatal checkups to stay informed about any changes to your estimated due date. And be prepared for delivery a few weeks ahead of time, just in case.
Pregnancy Due Date FAQs
Can ultrasound be wrong about due dates?
Keep in mind that all due dates are estimates. Ultrasound isn’t “wrong” when estimating due dates, but it can become less accurate the longer you wait to get one during your pregnancy, Thornburg says.
According to Thornburg, ultrasounds usually pinpoint due dates within:
- Five days of the actual delivery date when done before the ninth week of pregnancy
- Seven days of the actual date when done between the ninth and 16th week
- 10 days of the actual date when done before 22 weeks
The estimate might be off by more than two weeks if you wait until after the 22nd week to get an ultrasound.
“This is why it is critical to establish the due date early, and then why we do not change it after that,” Thornburg says
How am I four weeks pregnant if I conceived two weeks ago?
Your pregnancy is dated from the first day of your last menstrual period and not from the day of conception, which usually happens around day 14 of your cycle, Chelliah says. “When calculating pregnancy, we start counting from day one of your last period, even though you are not pregnant during those first two weeks,” she says.
Can you get a positive pregnancy test at three weeks?
Yes. When an embryo starts to attach to the uterus, it makes a hormone called beta-hCG. Some pregnancy tests can detect this hormone as early as eight days after ovulation (about three weeks after your last period if you have a 28-day cycle). But not all home tests can pick up these low levels of the hormone that early. Levels of beta-hCG increase and become more easily detectable as your pregnancy moves forward.
