Every pregnant woman wants to know her due date. But a due date calculated from last menstrual period with a due date calculator often doesn't match the due date that is estimated by ultrasound (also known as a sonogram).
During an ultrasound, a technician spreads a warm gel over the lower part of the abdomen, and then presses a tool called a transducer against the belly to examine the fetus using sound waves. An image of the fetus appears on an accompanying computer screen. While looking at this image, the technician takes some standard measurements from different angles and listens for a heartbeat.
While you may be more familiar with this trans-abdominal ultrasound, it is common for doctors to use a transvaginal ultrasound for pregnancy dating, especially early in the first trimester. Sonograms are risk-free, and seeing your baby will likely be an enjoyable experience.
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Your Baby's Ultrasound: What to Expect
Due Date Accuracy
Naturally, one of the most common questions asked about ultrasound is: Just how accurate are due dates predicted by ultrasound? Evidence suggests that, in the first 20 weeks of pregnancy, the first ultrasound may be the most accurate tool for calculating a fetus's gestational ages.
Due dates predicted with early ultrasound have a margin of error of roughly 1.2 weeks. Doctors will usually keep the original due date (the one generated by the date of the last menstrual period) if the ultrasound due date is within that margin of error.
If you can't remember the date of your last menstrual period date, an early ultrasound can give you a fairly reliable due date. Ultrasound dating is also helpful if your menstrual cycles are irregular.
When Due Dates Don't Match
If an early ultrasound gives you a due date that is more than 1.2 weeks away from what was expected, there is a good chance that everything is still fine. You may have simply conceived earlier or later than you thought you did (which can happen if your cycle is at all irregular or if you remembered your last menstrual period date incorrectly).
Your doctor might want to repeat the ultrasound to make sure that your pregnancy is developing as it should. Assuming that the subsequent ultrasound shows the expected fetal growth for the time period between the scans, your doctor may revise your due date to match the first ultrasound's predictions.
As long as repeat ultrasounds show consistent growth and no signs of problems, there is no reason to be concerned if your doctor changes your due date.
Ultrasounds in Later Pregnancy
It is common for each ultrasound throughout pregnancy to predict a different due date. Earlier ultrasounds are more accurate in terms of predicting the due date, so that's why doctors will usually use the dates and measurements from the first ultrasound of the pregnancy as a reference.
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. Thus, later in pregnancy, ultrasound measurements are more valuable for evaluating the baby's growth over time (compared with earlier measurements) than they are for predicting a due date.
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Barnhart KT, Simhan H, Kamelle SA. Diagnostic accuracy of ultrasound above and below the beta-hCG discriminatory zone. Obstet Gynecol. 1999;94(4):583-7. doi:10.1016/s0029-7844(99)00347-6
Mongelli M, Wilcox M, Gardosi J. Estimating the date of confinement: Ultrasonographic biometry versus certain menstrual dates. Am J Obstet Gynecol. 1996;174(1 Pt 1):278-81. doi:10.1016/s0002-9378(96)70408-8(Video) How accurate is ultrasound in dating a pregnancy?-Dr. Nupur Sood
By Krissi Danielsson
Krissi Danielsson, MD is a doctor of family medicine and an advocate for those who have experienced miscarriage.
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Is ultrasound accurate in determining due date? ›
Between 22 0/7 weeks and 27 6/7 weeks of gestation, ultrasonography dating has an accuracy of ± 10–14 days 19.Are ultrasounds 100% accurate? ›
The ultrasound scan isn't 100 per cent accurate, but the advantages of the test are that it's non-invasive, painless and safe for both mother and unborn baby. If fetal abnormalities are detected, you may be offered further tests to confirm the diagnosis, such as amniocentesis and chorionic villus sampling.What is the most accurate way to determine due date? ›
Instead, she says the most accurate method of calculation (aside from IVF transfer dates, which are the most accurate) is an early first trimester ultrasound and certain date of last menstrual period, followed by the date of conception and lastly, a second or third trimester ultrasound.How accurate are pregnancy ultrasounds? ›
A study published in 2016 found that first-trimester ultrasounds were capable of finding anomalies. The researchers were successful in finding birth defects in 30 percent of women at low risk and 60 percent of women at high risk of having babies with anomalies.Can ultrasound dates be wrong in pregnancy? ›
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.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.Why did my due date change after my ultrasound? ›
“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.What week ultrasound is most accurate? ›
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.Do you subtract 2 weeks from ultrasound? ›
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.How accurate are Obgyn due dates? ›
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. "The reality is that about 70% of women will have their baby within 10 days of their due date," he said.
How inaccurate can a due date be? ›
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.Can your due date be wrong? ›
A normal cycle can be between 21 to 35 days, and some people can even have cycles that range outside of that. Regularity isn't that common, so there's a fair chance that the calculation will be off, but the last menstrual period measurement is still used because it's considered fairly reliable most of the time.How often are ultrasounds inaccurate? ›
It's important to know there is a recognised 15% error in ultrasound fetal weight estimation. This means your baby can be either 15% smaller or 15% larger than the estimated weight given at your ultrasound. Ultrasound scans are generally more accurate for assessing your baby's size during the first half of pregnancy.Can you tell how far along you are by an ultrasound? ›
One of the biggest benefits of an ultrasound is that it can estimate how far along you are in your pregnancy (gestational age) down to just a few days. Ultrasounds estimate how far along you are in your pregnancy by comparing the growth of the fetus to established growth rates for babies around the world.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.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 LMP and ultrasound dates not match? ›
Clinical determination of EDD, 280 days after the last menstrual period (LMP) still plays a role but may not always be accurate due to variability in length of an individual woman's cycle length or timing of ovulation.What percentage of due dates are accurate? ›
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.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 an ultrasound be off by 2 weeks in first trimester? ›
Determining a Due Date
Evidence suggests that ultrasounds more accurately predict your due date than using your last menstrual period—but only in the first trimester and early second trimester (until roughly 20 weeks). 3 Early ultrasound due dates have a margin of error of roughly 1.2 weeks.
How accurate are doctors at predicting due dates? ›
Only 4 percent of women give birth on their estimated delivery date. That's because of the natural variation in how long it takes a baby to grow and because of our limited ability to predict due dates.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.How accurate are late third trimester ultrasounds? ›
CONCLUSION: Our estimated fetal weight measurements at 37 weeks and beyond may be considered accurate as our error rate is below the acceptable rate of <15%.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.How is due date determined? ›
An estimated due date can be calculated by following steps 1 through 3: First, determine the first day of your last menstrual period. Next, count back 3 calendar months from that date. Lastly, add 1 year and 7 days to that date.How accurate are due dates for first babies? ›
About half of first-time mothers will give birth by 40 weeks and 5 days after the first day of their last menstrual period, with the other half giving birth after that time point.Is it normal for ultrasound to be off by a week? ›
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.How do they measure how many weeks pregnant you are by ultrasound? ›
A dating ultrasound, which measures the length of the fetus from crown to rump, is an accurate way to estimate your due date or how far along you are if completed within the first trimester (the first 13 weeks of pregnancy).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.Can doctors determine the exact date of conception? ›
Sperm can live in a woman's body for up to five days, making it impossible to calculate exactly when conception occurred. It is not a fine science.
When is ultrasound most accurate for due date? ›
The earlier in the second trimester that an ultrasound is performed, the more accurate gestational age measurement. If a first-trimester ultrasound has been used to confirm or establish an estimated date of delivery, a second-trimester ultrasound should not be used to adjust the estimated date of delivery.Can ultrasound detect exact weeks of pregnancy? ›
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.Is blood test or ultrasound more accurate for due date? ›
The most accurate way to date a pregnancy is with an ultrasound done between 6 weeks and 8 weeks, 6 days, according to Dr.