Your due date is one of the first things that will be calculated in your prenatal care. The due date guides you and your provider in gestational age-appropriate prenatal care, testing, and ultimately your baby's birth. That's why it's important that your due date is as accurate as possible.
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However, it's not always straightforward. You might find yourself being told that your due date is a different date than you originally expected. In fact, as many as 26% of people have their due dates changed in pregnancy. Here are a few questions to ask your provider before having your due date changed.
Causes of Changing a Due Date
Your provider will look at several factors as your pregnancy progresses to ensure that your due date continues to be accurate. Some of the factors that they will use to determine the accuracy of your due date include:
- Fetal heart tones: When did a provider hear the baby’s heartbeat and which instrument did they use?
- Fundal height: The fundus (top of the uterus) might measure differently than expected based on your predicted due date.
- Quickening: When did you first feel the baby move?
Other factors, such as the discovery of a twin pregnancy, uterine anomaly, and your weight can also affect the calculation of your due date
Due Date Accuracy
One common scenario is that a person has been told that their due date is a certain date throughout their entire pregnancy, but then an ultrasound near the middle of the pregnancy, they are told a different date—usually, it's only a matter of days different.
This change, if it is less than 14 days in either direction, is probably within the normal variation. Ultrasounds at this point are limited for dating a pregnancy. The specificity of ultrasound dating a pregnancy varies with each trimester.
The most accurate dating is in the first trimester (which varies plus or minus seven days) and the least accurate dating is in the third trimester.
Your provider will recommend that you have an ultrasound during the first trimester of pregnancy to see if the due date calculated from the day of your last menstrual period needs to be adjusted.
Impact on Prenatal Care
An altered due date usually won't have an immediate effect on your prenatal care. The alteration potentially comes at the end of pregnancy when you might be looking to move toward delivery because of the new due date. If you do decide to go with an amended due date, this is something to remember if it comes time to start talking about induction of labor.
One of the best ways to avoid changes in due dates is to ensure that you have the most accurate data. When your provider looks at your due date derived from your last menstrual period, it's considered to be plus or minus 14 days. This means that your provider expects that your baby will show up between weeks 38 and 42 from the date calculated.
The average length of gestation is 266 days from the date of conception, which is reported as 280 from the last menstrual period. However, there is an issue with this measurement because it assumes that you ovulated on day 14, which might not always be true. If you do not know when you ovulated, you can consider using the length of your cycle and counting backward 14 days.
Naegle’s rule is used to calculate a due date. You take the first day of your period and count backward three months, and then add seven days.
For example, if your period started on February 1st, you would count back three months to November 1st and add seven days for a due date of November 8th.
For someone with a 32-day cycle, a good guess for the date of ovulation would be day 18. This would alter your due date by four days. While four days might not seem like a lot of time at the beginning of your pregnancy, it could translate to a lot of time toward the end of pregnancy when your provider might be trying to decide whether to induce labor or wait a few more days.
A Word From Verywell
Never hesitate to ask questions about a due date change. The information can only help you understand what is going on and why this is being discussed. It can help you move forward with the healthiest pregnancy possible.
Verywell Family uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
Declercq ER, Sakala C, Corry MP, Applebaum S, Herrlich A. Major Survey Findings of Listening to Mothers(SM) III: Pregnancy and Birth. J Perinat Educ. 2014;23(1):9-16. doi:10.1891/1058-1243.23.1.9
Jukic AM, Baird DD, Weinberg CR, McConnaughey DR, Wilcox AJ. Length of human pregnancy and contributors to its natural variation. Hum Reprod. 2013;28(10):2848-2855. doi:10.1093/humrep/det297
- Ananth, Cande V. (2007). Menstrual versus clinical estimate of gestational age dating in the United States: temporal trends and variability in indices of perinatal outcomes. Paediatric and Perinatal Epidemiology, 21, 22-30. doi: 10.1111/j.1365-3016.2007.00858.x
- Declercq ER, Sakala C, Corry MP, Applebaum S, Herrlich A. Listening to Mothers III: Pregnancy and Childbirth. New York: Childbirth Connection, May 2013.
- Gabbe SG , Niebyl JR, Simpson JL, Galan H, Goetzl L, Jauniaux ER, Landon M. (2007). Obstetrics: Normal and Problem Pregnancies (5th ed.): Churchill Livingstone.
- Goldenberg, Robert L., McClure, Elizabeth M., Bhattacharya, Anand, Groat, Tina D., & Stahl, Pamela J. (2009). Women's Perceptions Regarding the Safety of Births at Various Gestational Ages. Obstetrics & Gynecology, 114(6), 1254-1258.
By Robin Elise Weiss, PhD, MPH
Robin Elise Weiss, PhD, MPH is a professor, author, childbirth and postpartum educator, certified doula, and lactation counselor.
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What does it mean when your doctor changes your due date? ›
“If dating is only based on the last menstrual period and a later ultrasound shows a discrepancy, then the due date may be changed,” Lamppa says. If your due date is confirmed by an ultrasound in the first trimester, it shouldn't change as the pregnancy progresses, even with additional ultrasounds, she says.How far off can doctors be on your due date? ›
As pregnancy progresses, the accuracy of an ultrasound for predicting due dates decreases. Between 18 and 28 weeks of gestation, the margin of error increases to plus or minus two weeks. After 28 weeks, the ultrasound may be off by three weeks or more in predicting a due date.Is it possible for doctor to mistake due date? ›
It is very common when scanning an early pregnancy to find that the due date does not match the menstrual history. Sometimes the dates can be more than a week off and sometimes even as much as 4 weeks.How accurate are doctors at predicting due dates? ›
Even once your doctor has established a due date, there's no guarantee that your baby will come that day. In fact, only 6% of women deliver on their due date, Cackovic said. I tell patients that really, your due date is a date in the middle of almost a month-long period of time that you could have your baby.Will my due date change if baby is measuring 2 weeks ahead? ›
Ultrasound-determined due dates are most accurate during the first trimester, and measuring 'ahead' doesn't result in a changed due date in most cases.Can gestational age be off by 2 weeks? ›
After the infant is born, there are a variety of characteristics that can be used to estimate the gestational age. It is possible for gestational age to be inaccurate by up to 2 weeks, even with an accurate LMP date confirmed by other tests.Is it normal for due date to change? ›
You might find yourself being told that your due date is a different date than you originally expected. In fact, as many as 26% of people have their due dates changed in pregnancy.What causes a baby to be overdue? ›
You are more likely to be overdue if you are obese, have never given birth before or if you're over the age of 30. Your midwife or doctor will check that both you and your baby are healthy by giving you ultrasound scans and checking your baby's movement and heartbeat.How off can ultrasound due date be? ›
Between 22 0/7 weeks and 27 6/7 weeks of gestation, ultrasonography dating has an accuracy of ± 10–14 days 19.Why is my due date 2 weeks off? ›
To clarify, the gestational period of 40 weeks actually starts with the first day of your last period, which adds two weeks of time to the gestational period when your baby didn't even exist yet…
Can a due date be off by 6 weeks? ›
It typically lasts between 38 and 42 weeks. Then the doctor takes the average of those two time frames and adds it to the previous date and, voilà, your due date is determined. The problem is that this date can be wrong by up to 6 WEEKS!!! Your childbirth is not based on the average.Why my due date is different from the ultrasound? ›
This is because the gestational age is calculated from the first day of your last menstrual period (lmp) and the fetal age is based on your baby's size as measured during a scan. It's normal for some difference between the gestational age and fetal age to exist, as babies develop in a normal range of shapes and sizes.Can my due date change in third trimester? ›
The third trimester is the least accurate time to date a pregnancy. Estimates based on an ultrasound can be off by as much as three weeks, so doctors rarely adjust dates during the third trimester.What percentage of due dates are correct? ›
Only about 5% of babies will arrive on their due date and focusing on this single date can make the end of pregnancy quite stressful. It may be more helpful to be prepared for you baby arrive some time after 37 weeks, and to focus on 42 weeks as the time by which you have a good chance of having given birth.Which scan date is more accurate? ›
The results showed that in over 50% of cases the estimated date of delivery derived from the ultrasound scan was more accurate than that derived from the last menstrual period, whatever the discrepancy between the two predictions.Why is my baby measuring a week ahead on ultrasound? ›
Measuring your fundal height is one way your healthcare provider can make sure the fetus's growth is on track. If you're told the fetus measures ahead or behind schedule, try not to worry. It usually just means an ultrasound or further testing is needed. The fetus is more than likely developing perfectly.Why do doctors add 2 weeks to pregnancy? ›
Your weeks of pregnancy are dated from the first day of your last period. This means that in the first 2 weeks or so, you are not actually pregnant – your body is preparing for ovulation (releasing an egg from one of your ovaries) as usual.Why is my baby measuring 3 days ahead? ›
Most of the time, there's a harmless explanation. Maybe your due date is off by a few days or a week (it's pretty common for doctors to change due dates). Your baby could be in a funny position or sitting high in your uterus, and that's throwing the tape measure off.How far off can ultrasound measurements be? ›
Ultrasound is not very reliable for estimating fetal weight near term. For a 9-pound baby, an ultrasound's predictive accuracy is typically 15 to 20 percent off. Which means we may over- or underestimate by more than a pound.What if ultrasound shows less weeks? ›
If the measurement is low for the number of weeks, then the baby may be smaller than expected. Other tests used for diagnosis may include: Ultrasound to estimate the baby's size. Doppler flow to help check blood flow to the baby during pregnancy.
Can early ultrasounds be off by a week? ›
Ultrasound dating of conception is not reliable for determining paternity because the test can be off by at least 5-7 days in early pregnancy.How many days is normal to go over due date? ›
Most doctors and midwives are happy for you to go a few days over your due date as long as everything seems to be okay. Many will let pregnant women go up to two weeks over. After 42 weeks, however, the baby's health might be at risk.What is the longest a baby has been overdue? ›
- 30 facts about pregnancy. The longest recorded pregnancy was 375 days. According to a 1945 entry in Time Magazine, a woman named Beulah Hunter gave birth in Los Angeles nearly 100 days after the average 280-day pregnancy. ...
- 5 myths. Myth: The shape of your belly can predict the gender of your baby.
After studying the data of various moms and their birth dates, researchers found a connection between high stress events and women having longer-than-expected pregnancies. Pregnant women that were 33-36 weeks along on September 11, 2001 were found to have more post-term pregnancies than expected.Are Overdue babies more developed? ›
Researchers found that babies born at late term—41 weeks' gestation—are slightly more likely to be classified as gifted and have higher standardized test scores than babies born at full term, or at 40 weeks' gestation.How often are due dates wrong? ›
The chance of delivering exactly on your magically assigned date is rare — only 5% of women will deliver on their Estimated Due Date. So there's a pretty good chance that your due date is wrong too. But why are we so bad at accurately predicting due dates?Can an ultrasound give you the exact date of conception? ›
Fetuses grow quickly, and they also follow the same pattern of growth in virtually every case. That means that by measuring the size of the fetus and looking for certain distinguishing characteristics, an ultrasound can give an accurate estimate of when you conceived.Should I take a week off before my due date? ›
When should I take my maternity leave? Some women begin taking their leave a week to a month before the expected birth because of discomfort or the desire for time to prepare. Others wait until the last moment so they can maximize their time with the baby once it arrives.How many weeks is considered to be overdue? ›
Between 41 weeks and 41 weeks and six days, a pregnancy is called late-term. When a pregnancy reaches 42 weeks and beyond, it's postterm. Late-term and postterm pregnancy can raise the risk of some health problems, including: Larger than average birth size (fetal macrosomia).What week is most common to go into labor? ›
- 57.5 percent of all recorded births occur between 39 and 41 weeks.
- 26 percent of births occur at 37 to 38 weeks.
- About 7 percent of births occur at weeks 34 to 36.
- About 6.5 percent of births occur at week 41 or later.
- About 3 percent of births occur before 34 weeks of pregnancy.
What happens if ultrasound does not match LMP? ›
LMP versus early ultrasound
If an ultrasound date in the first trimester differs from your LMP date by seven days or more, we would go with the ultrasound. If the ultrasound date is within seven days of your LMP date, we would stick with your LMP date.
Speak to your midwife or birthing care provider to understand what they based your due date on. Ask them if they took into account the length of your menstrual cycle and your ovulation cycle. Find out which due date calculation method they used.What is the most common due date? ›
- 10% gave birth by 38 weeks and 5 days after the LMP.
- 25% gave birth by 39 weeks and 5 days after the LMP.
- 50% gave birth by 40 weeks and 5 days after the LMP.
- 75% gave birth by 41 weeks and 2 days after the LMP.
First-trimester ultrasound (ultrasound before 13 weeks and 6/7 days) is the most accurate method to establish or confirm gestation age in pregnancy. First-trimester ultrasound can be performed either trans-vaginally or trans-abdominally.Are first babies usually late? ›
First babies are less likely to be “on time” at 39 weeks, and more likely to be a little late, between 41 and 43 weeks. Among full-term pregnancies, first babies are born about 1.3 days later on average.How accurate is the due date of a baby? ›
More than 90% are born two weeks either side of the predicted date. But, as noted above, only 4% (or 4.4%, ignoring pregnancies with complications etc) are born on the predicted date itself - in other words, the chance of this happening is less than one in 20.Which due date is most accurate? ›
Ultrasound measurement of the embryo or fetus in the first trimester (up to and including 13 6/7 weeks of gestation) is the most accurate method to establish or confirm gestational age.How long after a due date will a doctor induce labor? ›
Your provider may recommend inducing labor if your health or your baby's health is at risk or if you're 2 weeks or more past your due date. For some women, inducing labor is the best way to keep mom and baby healthy. Inducing labor should be for medical reasons only.What is more accurate ultrasound or due date? ›
An ultrasound is actually the most accurate way to date a pregnancy because all fetuses grow at a consistent rate during the first trimester and early second. In other words, if your baby measures 9 weeks 2 days when you have your ultrasound, that's how far along you are, no matter when your last period was.Why does ultrasound show different due date? ›
This is because the gestational age is calculated from the first day of your last menstrual period (lmp) and the fetal age is based on your baby's size as measured during a scan. It's normal for some difference between the gestational age and fetal age to exist, as babies develop in a normal range of shapes and sizes.
Is it common for due dates to change? ›
You might find yourself being told that your due date is a different date than you originally expected. In fact, as many as 26% of people have their due dates changed in pregnancy.Why was my due date pushed back? ›
It could be that your uterus size doesn't sync up with the standard growth charts (i.e. you measure big). And if the measurements are off by three weeks or more, in many cases your due date is moved to reflect this.Is it better to be induced or wait? ›
Can I wait for labor to begin naturally? Nature typically prepares the cervix for delivery in the most efficient, comfortable way. However, when there's concern about mother's or baby's health or the pregnancy continues two weeks past the due date, inducing labor might be the best option.What week is best to induce labor? ›
When a woman and her fetus are healthy, induction should not be done before 39 weeks. Babies born at or after 39 weeks have the best chance at healthy outcomes compared with babies born before 39 weeks. When the health of a woman or her fetus is at risk, induction before 39 weeks may be recommended.Is it normal to have a baby 2 weeks early? ›
"The risk to any individual baby born two to three weeks early is only about 4 percent, so we don't want parents to be alarmed," said study author Dr. Steven Morse, director of Community Newborn Services at the University of Florida.How do I know the exact date I got pregnant? ›
A reverse due date calculator is helpful if you already know your due date and you want to find out when you got pregnant. To find out when you conceived, just subtract 266 days from your expected delivery date. You probably conceived around that day.Why do doctors make you wait 6 weeks? ›
Your doctor will be making sure that you are healing as expected. By 6 weeks postpartum, your uterus should also have returned to its normal size – about the size of a grapefruit. If you are due for a pelvic exam, or if you had a complicated delivery or episiotomy, you can expect a pelvic exam.