Due date calculator by ultrasound (2024)

Updated 01 August 2024 |

Published 03 April 2019

Medically reviewed by Dr. Allison K. Rodgers, Reproductive endocrinologist, infertility specialist, obstetrician, and gynecologist, Fertility Centers of Illinois, Illinois, US

Written by Rhalou Allerhand

Please note that Flo Health does not collect, process, or store the data that you enter while using these Tools. All calculations are done exclusively in your browser. Flo Health does not have access to the results. All data will be permanently erased after leaving or closing the page.

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Week 1

At 1 week pregnant, you’re actually not pregnant yet. As your pregnancy is calculated from the first day of your last menstruation, your baby does not yet exist, and your body is preparing for the ovulation during which you’ll get pregnant.

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Week 2

At 2 weeks pregnant, you’re technically not pregnant yet. Right now there is a lone egg and a whole bunch of anxious sperm eager to fertilize the egg. Your uterus and the entire body are preparing for a big day of ovulation - the stage when you'll get pregnant.

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Week 3

Week 3 of pregnancy is the week when the implantation happens. Your body releases chorionic gonadotropin (hCG), which causes an increased production of estrogen and progesterone, and prevents new eggs in the ovaries from ripening. Very soon you'll start experiencing the first symptoms of pregnancy: missed period, nausea, breast changes.

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Week 4

At 4 weeks pregnant, your baby is the size of a poppy seed.

At 4 weeks pregnant, your future baby has finally found his home for the next eight months. The blastocyst has arrived from a fallopian tube to your uterus. You can get a positive pregnancy test result at this stage.

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Week 5

At 5 weeks pregnant, your baby is the size of a sesame seed.

By week 5, you should have missed your period, which is one of the most obvious signs you're expecting. Under the influence of hormonal changes, you can feel the first signs of pregnancy: breast swelling, fatigue, headache, and back pain.

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Week 6

At 6 weeks pregnant, your baby is the size of a lentil.

Starting from pregnancy week 6, you may experience morning sickness. This is the result of hormonal changes occurring in your body. Malaise, breast swelling, darkening of the nipple areola, and frequent urination can bother you, too. In case of bleeding, you should consult your doctor.

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Week 7

At 7 weeks pregnant, your baby is the size of a blueberry.

At 7 weeks pregnant, symptoms start kicking in and your uterus almost doubles in size. Be prepared for a possible increase in nausea, fatigue, heartburn, and other pregnancy symptoms. Morning sickness may give a lot of trouble. Try to find some ways to cope with it.

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Week 8

At 8 weeks pregnant, your baby is the size of a bean.

At 8 weeks pregnant, you need to plan your first visit to the gynecologist. The doctor will prescribe the necessary tests and examinations for the first trimester of pregnancy. You may feel the growing discomfort of morning sickness. Try to be patient; it usually lasts until the 14th week only.

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Week 9

Week 24

At 24 weeks pregnant, your baby is the size of an eggplant.

At 24 weeks pregnant, your baby is almost a foot long. You could be experiencing a tingling sensation in your joints, which is known as carpal tunnel syndrome. It is a common condition during pregnancy which occurs due to fluid build-up in your joints which results in compression of the median nerve.

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Week 25

At 25 weeks pregnant, your baby is the size of a full ear of corn.

Once you reach week 25 of your pregnancy, you’ll be nearing the end of your second trimester. It can feel like times flies! At 25 weeks pregnant, you’re approximately 5 months and 2 weeks along. Your baby has been growing steadily and even though it’s still not ready, it won’t be long before it comes into the world.

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Week 26

At 26 weeks pregnant, your baby is the size of a beetroot.

You’re likely to put on between 16 and 22 pounds by now. At one point during this week, your baby will open his or her eyes for the first time. He or she is not yet able to see anything inside of the uterus but will blink closing and opening his or her eyes when falling asleep and waking up.

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Week 27

At 27 weeks pregnant, your baby is the size of a cauliflower.

The 27th week of the pregnancy marks the final two weeks of the second trimester. If your baby is more active at night you might suffer from insomnia and have trouble sleeping. Compensate for the lack of sleep time during the night by napping during the day more when the baby is sleeping.

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Week 28

At 28 weeks pregnant, your baby is the size of a coconut.

At 28 weeks you are now entering the third trimester of your pregnancy. At this stage, your baby is pretty well-developed. Her organs, tissues, and nerves continue to grow, but she already has all of the systems necessary for survival outside the uterus. Towards the end of the pregnancy, babies start to recognize familiar sounds and voices.

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Week 29

At 29 weeks pregnant, your baby is the size of a pomelo.

At 29 weeks pregnant, you're likely to develop varicose veins like 40 percent of expectant moms. It's also a good time to start doing a kick count. Let your doctor or midwife know if you notice that your baby is becoming less active.

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Week 30

At 30 weeks pregnant, your baby is the size of a head of cabbage.

At 30 weeks pregnant, you are likely to experience shortness of breath. Your baby is still up high near your rib and is waiting a bit – it is soon expected to drop down into your pelvis.

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Week 31

At 31 weeks pregnant, your baby is the size of a head of a zucchini.

At 31 weeks pregnant, your breasts can get leaky producing the first baby’s food – colostrum. This is one of the symptoms that your body is getting ready for the big day. You are likely to experience shortness of breath. This week your baby is going through major nerve and brain development.

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Week 32

At 32 weeks pregnant, your baby is the size of a head of lettuce.

At 32 weeks pregnant, your body may start flexing its muscles preparing for the big day. Your baby is also preparing for her debut mastering the skills she’ll need to thrive outside your womb: swallowing, breathing, sucking.

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Week 33

At 33 weeks pregnant, your baby is the size of a pineapple.

At 33 weeks pregnant, you may notice that your baby’s movements are affected by your daily routine. Your belly continues to grow and it’s getting even more troublesome to find a comfortable sitting or sleeping position.

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Week 34

At 34 weeks pregnant, your baby is the size of a butternut squash.

At 34 weeks pregnant, your breasts could start leaking small amounts of yellowish colostrum. Your baby is already the size of a school bag and weighs as a melon. If you’re worried about your safety at work, time to talk to your employer about maternity benefits.

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Week 35

At 35 weeks pregnant, your baby is the size of a honeydew melon.

At 35 weeks pregnant, you may know how your baby’s moving in your womb just by looking at your bump. It can you give you some discomfort and make you a bit breathless. At this point, many moms can’t wait for the baby to get here, while others are feeling a bit anxious about giving birth. Both feelings are completely normal!

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Week 36

At 36 weeks pregnant, your baby is the size of a papaya.

At 36 weeks pregnant, your baby is sleeping between 60 and 80% of the time. It has finally moved into your pelvic cavity, the pressure on your diaphragm is released, and lightening happens. Your baby can now open its eyes, suck its thumb, breathe, and recognize voices!

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Week 37

At 37 weeks pregnant, your baby is the size of a head of romaine lettuce.

Welcome to your 37th week of pregnancy, and congratulations! The baby moves further into the pelvis. It is considered to be ‘at-term’ and can actually arrive any day now. Make sure you are ready for the arrival of a new family member.

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Week 38

At 38 weeks pregnant, your baby is the size of Swiss chard.

At 38 weeks pregnant, you can find yourself spending the whole life peeing. The pressure on your bladder is tremendous. Your baby is a fully functioning little human and your placenta is fully grown.

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Week 39

At 39 weeks pregnant, your baby is the size of a small watermelon.

Welcome to the week 39 of pregnancy! Your baby is full term, meaning that it is fully developed and is only waiting for the right time to make an entrance into the world. Have you prepared everything that is needed to welcome your baby?

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Week 40

At 40 weeks pregnant, your baby is the size of a mini watermelon.

At 40 weeks pregnant, you may feel disappointed that your due date has come and gone. Don’t panic and make the last preparations for a new human who’ll soon join the world.

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Week 41

At 41 weeks pregnant, your baby is the size of a pumpkin.

At week 41 of pregnancy, you might be dying out of the desire to give birth and see your baby. But rest assured that plenty of moms-to-be go past their due date and everything turns out just fine.

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Week 42

At 42 weeks pregnant, your baby is the size of a watermelon.

When a pregnancy lasts for 42 weeks or more it is referred to as a post-term pregnancy. While not many studies exist that prove why some women’s pregnancy lasts for 42 weeks, medical experts believe that factors such as hormones, genetics, and even obesity can be the cause.

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Just taken a positive pregnancy test? Once the exciting news has sunk in, you’ll no doubt be eager to find out when your baby is due to arrive, also known as your estimated due date (EDD).

Even if pregnancy symptoms have kicked in, it’s normal to feel like your pregnancy doesn’t officially start until you actually get to see your little baby on the screen during your first ultrasound. Fortunately, it won’t be too long before it’s time for your first scan.

One common way of figuring out your EDD is to use the date of your last period, but it’s also possible to calculate your due date another way: using your ultrasound. Keep reading to hear more about how that’s done. But if you can’t wait until your appointment, and you’re curious to find out what your due date might be according to your ultrasound, this tool could give you a rough idea.

By inputting the date of your scan into our due date by ultrasound calculator above and adding how far through your pregnancy you think you are, it’ll do the math on when your baby should be ready to make its arrival into the world. Remember, though, it's just an estimate and is not a replacement for medical advice. Then scroll down for everything you need to know about gestational age (the term doctors use to describe how many weeks pregnant you are) and how ultrasounds work, as well as what you can expect at the scan.

Your due date is just an estimate of your baby’s arrival and not a guaranteed birthday, so be prepared for your baby to arrive any time as you approach the end of pregnancy. The average length of a pregnancy is 40 weeks, but only 5% of babies arrive on their exact due date. It’s perfectly normal for babies to arrive anywhere between 37 and 41 weeks of pregnancy, so get that hospital bag packed.

Knowing how far along you are in your pregnancy is important for lots of reasons. Tracking your baby’s development can help you figure out what to expect at every stage of your pregnancy. Plus, your baby will hit lots of important milestones before their delivery date. So a due date gives your health care provider the essential information they need to monitor the health of your growing baby, too.

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Key takeaways

Your due date is estimated using a combination of an ultrasound scan and the date of your last menstrual period. The calculator above uses your ultrasound scan to predict a due date.
Your baby’s due date is an informed guess and not a guaranteed birthday. Remember, only 5% of babies arrive on their exact due date.
It’s common for your due date to change because your last period date may not align with your ultrasound due date by days or even weeks.
Flo Premium’s pregnancy mode can help you and your partner (if they’re using Flo for Partners) learn about all the exciting milestones you’ll reach in the journey to your due date. You can also use Flo’s safe community space, Secret Chats, to chat with others going through their pregnancy journey.

Is your due date based on your last period or ultrasound?

You may have landed on this due date calculator if you’ve recently discovered you’re pregnant, but you’re confused how to figure out your due date. We’ll break it down for you with a little help from obstetrician and gynecologist Dr. Renita White, Georgia, US.

Admittedly, figuring out your due date can be a little complicated, because there are a couple of different ways to do it. In general, pregnancies last for 280 days (or 40 weeks). But it’s normal for them to last anywhere between 37 to 41 weeks, meaning that depending on when you go into labor, it can be slightly longer or shorter than the 9 months you might have expected. To track pregnancy and calculate a due date, doctors use gestational age. Simply put, this is how long a person is pregnant in weeks and days. The gestational age of your baby is measured from your last menstrual period (LMP) — the first day of your last period — to the current date.

But this assumes you have a regular menstrual cycle of 28 days and that you always ovulate around day 14. Lots of us don’t. In fact, only 16% of women have a cycle lasting 28 days, and it can span anywhere from 21 to 35 days and still be considered normal. If you’re not sure when your period started or you have an irregular cycle, it can be pretty difficult to determine a due date in this way. What all this means is that your EDD may not always be entirely accurate, which is where a due date calculated by ultrasound can be more helpful.

You will get a clearer picture of your baby’s development when you have your first ultrasound scan at around 12 weeks or earlier (this can differ depending on where in the world you live). By measuring the size of your baby during this scan — more on how that works below — your doctor will build up a picture of how far through your pregnancy you are and when the baby will be due. They can then compare this estimate with the suggested due date from your LMP.

“For those who have a predictable monthly menstrual cycle, the estimated due date is based on the last period, as long as the ultrasound matches this date, with a five-to-seven-day margin of variation,” Dr. White says.

“If the ultrasound measurements are not consistent with the dates based on the first date of the last period, then the ultrasound due date would be used,” she adds.

How do you calculate the due date from a scan?

Your ultrasound scans are an important part of your pregnancy journey. Along with estimating your due date, they can also help your doctor to check on the health of your baby. So how do they work?

“An ultrasound scan is an imaging study that allows your health care provider to visualize the growing baby and the details of the baby’s body,” explains Dr. White. Most people have two ultrasound scans during pregnancy. There’s one toward the end of your 1st trimester that helps confirm your EDD (called a dating scan), and another at around 20 weeks (called an anatomy scan) to check that your baby is growing properly and to tell you their sex if you want to know. But you may have more if your doctor feels it’s necessary.

Your 1st-trimester scan will either be abdominal (a probe used across your midsection) or transvagin*l (a wand-like probe inside your vagin*). Neither should cause you any pain or any harm to your baby, but be sure to ask your health care provider if you have any questions.

You’re probably most familiar with the idea of an abdominal scan. If you have one of these, you may be asked to make sure you have a full bladder to help improve the angle of the fetus. This is usually the only part of the procedure that may cause a bit of discomfort. You’ll lie on a bed and pull up your top to expose your belly, before the person doing the scan — called a sonographer — smears gel on your abdomen. This might feel a little cold but shouldn’t hurt in any way. They’ll then move a probe across your belly to get a picture of your baby, occasionally pushing down in places to try to get a clear scan.

“This imaging uses high-frequency waves rather than radiation, so it is safe to be done in pregnancy,” Dr. White adds. A transvagin*l scan uses the same approach, so that’s completely safe, too.

The doctor will calculate your due date in slightly different ways depending on when you have the scan.

It’s most likely that you will have your dating scan in the 1st trimester. In this case, “The due date is determined by measuring the length between the top of the head to the bottom of the fetus. This is called the crown–rump length,” explains Dr. White.
If for some reason your dating scan happens in the 2nd trimester, your sonographer will “determine the due date and weight by measuring the size of the head, abdomen, and fetal femur bone,” she adds.

Why you might not see a baby in your ultrasound scan

If your health care provider can’t see your baby during your first ultrasound scan, don’t panic. One of the simplest explanations is timing, so it doesn’t necessarily mean there is anything wrong. “If you get an ultrasound too early, you might not see the growing baby yet,” says Dr. White. “The earliest you can see the fetus is around 6 weeks.”

That said, an ectopic pregnancy, when the embryo develops outside the uterus, or an early miscarriage can also affect your ultrasound scan results, so talk to your doctor if you’re concerned. They’re on hand to give you the support and advice you need.

When is an ultrasound most accurate for a due date?

The earlier in pregnancy your due date is determined using ultrasound, the more accurate it is likely to be, says Dr. White. “When a fetus is first growing (in the first 12 weeks), the size from crown to rump is fairly consistent among all fetuses,” she explains. However, as pregnancies progress, babies start developing at different rates. “Some will grow faster and gain more fat and length,” says Dr. White. This can make it “harder to get an accurate due date the further you go into the pregnancy,” she adds.

Can an ultrasound be off by two weeks in the 1st trimester?

It’s quite common for your due date to be tweaked by several days (or even weeks) if the EDD taken from your last period date doesn’t align with your ultrasound due date. Lots of people get their period dates mixed up or have an irregular cycle, both of which can affect a due date calculated by LMP.

The general rule is that “the earlier an ultrasound is done to confirm a due date, the more accurate [it is],” says Dr. White. So, “If the ultrasound is measuring more than five days off, when the last menstrual period should make someone 9 weeks or less, then ultrasound dating should be used,” she explains.

However, if for some reason your dating ultrasound is taking place later on in your pregnancy, for example in the 2nd or 3rd trimester, the EDD according to measurements taken in your scan is usually considered less accurate. This is because babies grow at different rates the further into a pregnancy you get. Ask your doctor if you have any questions about your due date or if it’s been changed and you’re not sure why.

It’s worth remembering that estimated due dates are just that — an estimate, says Dr. White. “They are rarely the exact date that someone will actually deliver,” she explains. “However, this date is an important reference point. It helps to determine if someone is full term versus preterm. It also helps to determine if babies are growing too small (or large) for their gestational age.”

It’s also important to remember the difference between due dates and delivery dates, adds Dr. White. “Based on someone’s pregnancy course, a recommendation may be given to deliver earlier than the due date,” she says. “This doesn’t mean the due date is changing. Rather, the due date stays the same, but the delivery date doesn’t align with the due date.”

References

“Abnormal Uterine Bleeding.” The American College of Obstetricians and Gynecologists, Dec. 2021, www.acog.org/womens-health/faqs/abnormal-uterine-bleeding.

“Definition of Term Pregnancy.” The American College of Obstetricians and Gynecologists, Nov. 2023, www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2013/11/definition-of-term-pregnancy.

“Ectopic Pregnancy.” Mayo Clinic, 12 Mar. 2022, www.mayoclinic.org/diseases-conditions/ectopic-pregnancy/symptoms-causes/syc-20372088.

Grieger, Jessica A., and Robert J. Norman. “Menstrual Cycle Length and Patterns in a Global Cohort of Women Using a Mobile Phone App: Retrospective Cohort Study.” Journal of Medical Internet Research, vol. 22, no. 6, June 2020, https://doi.org/10.2196/17109.

“How Long Does Pregnancy Last?” The American College of Obstetricians and Gynecologists, Nov. 2023, www.acog.org/womens-health/experts-and-stories/ask-acog/how-long-does-pregnancy-last.

Khambalia, Amina Z., et al. “Predicting Date of Birth and Examining the Best Time to Date a Pregnancy.” International Journal of Gynaecology and Obstetrics, vol. 123, no. 2, Nov. 2013, pp. 105–09, https://doi.org/10.1016/j.ijgo.2013.05.007.

MacKenzie, Andrew P., et al. “Prenatal Assessment of Gestational Age, Date of Delivery, and Fetal Weight.” UpToDate, 13 Oct. 2022, pro.uptodatefree.ir/Show/5391.

“Methods for Estimating the Due Date.” The American College of Obstetricians and Gynecologists, May 2017, www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date.

Morgan, John A., and Danielle B. Cooper. “Pregnancy Dating.” StatPearls, StatPearls Publishing, 12 Sep. 2022, www.ncbi.nlm.nih.gov/books/NBK442018/.

“Ultrasound In Pregnancy.” Cleveland Clinic, my.clevelandclinic.org/health/diagnostics/9704-ultrasound-in-pregnancy. Accessed 1 Aug. 2024.

History of updates

Current version (01 August 2024)

Medically reviewed by Dr. Allison K. Rodgers, Reproductive endocrinologist, infertility specialist, obstetrician, and gynecologist, Fertility Centers of Illinois, Illinois, US

Written by Rhalou Allerhand

Due date calculator by ultrasound (2024)

Take a quiz

Key takeaways

Is your due date based on your last period or ultrasound?

How do you calculate the due date from a scan?

Why you might not see a baby in your ultrasound scan

When is an ultrasound most accurate for a due date?

Can an ultrasound be off by two weeks in the 1st trimester?

References

History of updates

Current version (01 August 2024)

Published (03 April 2019)

References