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.
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.
LMP versus early ultrasound
If the ultrasound date is within seven days of your LMP date, we would stick with your LMP date. Ultrasounds done later in the pregnancy are less accurate for dating, so if your due date is set in the first trimester, it shouldn't be changed.
“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.
Some women with regular cycles are confused about why their ultrasound due date doesn't match up with their last menstrual period due date. Ovulation isn't a perfect science and can happen earlier or later than the norm, which might shift your due date slightly.
Between 22 0/7 weeks and 27 6/7 weeks of gestation, ultrasonography dating has an accuracy of ± 10–14 days 19.
It is possible for gestational age to be inaccurate by up to 2 weeks, even with an accurate LMP date confirmed by other tests.
20.1.
ultrasound performed between 6 and 13 weeks pregnancy: if the two dates differ by 5 days or less, use the LMP estimate; if the dates differ by more than 5 days, use the ultrasound estimate.
The ultrasound technician measures the “crown to rump” (head to buttocks) length of the embryo/fetus and will give you the gestational age. To translate into fetal age and estimate when you conceived, take the gestational age and subtract two weeks.
Abstract. Reported last menstrual period (LMP) is commonly used to estimate gestational age (GA) but may be unreliable. Ultrasound in the first trimester is generally considered a highly accurate method of pregnancy dating.
If you are unsure of LMP and/or there is a large discrepancy between your LMP and an early 1st trimester ultrasound, we will usually use the ultrasound date. 1st trimester ultrasounds are most accurate between 6-12 weeks.
A discrepancy between gestational age estimated by last menstrual period and biparietal diameter may indicate an increased risk of fetal death and adverse pregnancy outcome.
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.
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.
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.
Ultrasound is the most reliable method to establish pregnancy dating, particularly first-trimester ultrasound[1].
The most accurate way to date a pregnancy is with an ultrasound done between 6 weeks and 8 weeks, 6 days, according to Dr. Michael Cackovic, MD, an OB-GYN with The Ohio State University Wexner Medical Center.
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.
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.
Some reasons the fetus may measure ahead or large for gestational age are: Your due date is wrong. You have uterine fibroids. You have a BMI of over 25 (have overweight or obesity).
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.
A dating scan is an ultrasound scan done between 8 and 14 weeks of pregnancy to help estimate your baby's due date. In early pregnancy, most babies of the same gestational age are about the same size.
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.
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.