Last menstrual period ultrasound dating
Estimates based on an ultrasound can be off by as much as three weeks, so doctors rarely adjust dates during the third trimester.
However, it’s not uncommon for a doctor to perform an ultrasound in the third trimester if they’re thinking about changing your date.
Naegele’s rule involves a simple calculation: Add seven days to the first day of your LMP and then subtract three months.
For example, if your LMP was November 1, 2017: In this example, the due date would be August 8, 2018.
When you line up that date with the indicator, the wheel displays your due date.
Remember that the due date is only an estimate of when you will deliver your baby.
The other way to calculate your due date is to use a pregnancy wheel. It’s very easy to estimate your due date if you have access to a pregnancy wheel.
This is most likely to occur in the first trimester, especially if the date estimated by the ultrasound differs by more than one week from the date estimated by your doctor based on your LMP.
This is why ultrasound measurements can’t be used to accurately predict the age of the baby in the later stages of pregnancy.
Ultrasounds are not a necessary part of prenatal care.
The chances of actually having your baby on that exact date are very slim.
Some women have cycles that are consistently longer than the average 28-day cycle.