Last menstrual period ultrasound dating
Your doctor may change your due date if your fetus is significantly smaller or larger than the average fetus at your particular stage of pregnancy.
Generally, your doctor orders an ultrasound to determine the gestational age of your baby when there’s a history of irregular periods, when the date of your LMP is uncertain, or when conception occurred despite oral contraceptive use.
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.
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A repeat ultrasound provides valuable information about the growth of the fetus and may reassure you and your doctor that the change in due date is reasonable.
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 the time from ovulation to the next menstrual period.
If your cycle is 35 days long, for example, then you probably ovulated on day 21.
Pregnancy lasts an average of 280 days (40 weeks) from the first day of your last menstrual period (LMP).
The first day of your LMP is considered day one of pregnancy, even though you probably didn’t conceive until about two weeks later (fetal development lags two weeks behind your pregnancy dates).