Between 22 0/7 weeks and 27 6/7 weeks of gestation, ultrasonography dating has an accuracy of ± 10–14 days 19.
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.
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.
Third-trimester ultrasound (beyond 28 0/7 weeks) is the most inaccurate method for pregnancy dating with an accuracy of +/- 21-30 days[1]. One major concern with third trimester dating ultrasound is underestimating the gestational age of a growth-restricted fetus[1].
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.
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.
SYNOPSIS: 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.
“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.
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.
It is currently recommended that if only one ultrasound is performed, that it should be done at 18 to 20 weeks of gestation.
It is possible for gestational age to be inaccurate by up to 2 weeks, even with an accurate LMP date confirmed by other tests.
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.
Because of these problems with third trimester ultrasounds, the American College of Obstetricians and Gynecologists states that due dates should only be changed in the third trimester in very rare circumstances (2017).
You may have heard that these growth ultrasounds are not incredibly accurate, and the truth is that even when the measurements are extremely precise, the estimated weight can be different from the baby's true size by +/- 1 lb.
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.
The chances of an error with ultrasound are up to 5 percent, says Schaffir. An ultrasound can be between 95 to 99 percent accurate in determining sex, depending on when it's done, how skilled the sonographer is and whether baby is in a position that shows the area between their legs. Mistakes can also be made.
This is because pregnancy is counted from the first day of the woman's last period, not the date of conception which generally occurs 2 weeks later, followed by 5 to 7 days before it settles in the uterus.
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.
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.
If your menstrual cycle is longer, this means you ovulated later in your cycle and that your estimated due date will be later. If your menstrual period is shorter, this means that you ovulated earlier in your cycle and that your estimated due date will be earlier.
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.
Results indicate first trimester report of LMP reasonably approximates gestational age obtained from first trimester ultrasound, but the degree of discrepancy between estimates varies by important maternal characteristics.
Your doctor will use the images from the scan to measure the size of the fetus and determine how far along your pregnancy is. Once you know your baby's gestational age, subtract two weeks to estimate your date of conception.