The scan will usually show that your pregnancy is less far along than suggested by your LMP.
If you do know the conception date, this is counted as day 14, not day one.
If your cycle is irregular or you've recently been on the pill, working out the date from your LMP probably won't work. If the scan date differs from your period dates, the scan date will be used.
Add 7 days, This rule doesnt take into account the fact that many women are uncertain of the date of their last menstrual period, not all women have 28 day cycles, and not all women ovulate on day 14 of their cycle., et al) re-examined this issue for modern American women.
Results indicated that, for first-time Caucasian moms, an average pregnancy lasts 274 days from conception (approximately 288 days from the last menstrual period).
This research and other studies also show us many other factors influence the length of pregnancy, including: mothers age, weight, ethnicity, prenatal care, prenatal nutrition and smoking, number of prior pregnancies, and more.
Those calculations predict a due date based on the date of conception., who determined that the average human pregnancy was 266 days from conception, or 280 days (40 weeks) from the start of the last menstrual period.To calculate this, begin with the first day of your last period (LMP).If the date of conception and the date of the last menstrual period are unknown, there are several other ways to determine how far along the pregnancy is.Ultrasound: Ultrasound can be used to examine the babys development, and determine his/her age based on which systems are fully developed. By the third trimester, an ultrasound date can be off by as much as 3 weeks in either direction. The point of the scan is to work out how many weeks pregnant you are. Ectopic pregnancy and miscarriage: diagnosis and initial management CG 154.