During a pelvic exam, the uterus can be felt coming over the pubic bone at about 12 weeks’ LMP and at the umbilicus (navel) at about 20 weeks.The uterus then typically rises above the navel at about a centimeter a week after that.The body is not on a clock counting down to delivery; natural variation will introduce uncertainty.Health-care providers define the stage or length of pregnancy differently than many people might think. Logically, one might imagine we represent a pregnancy by how much time has elapsed since conception.Medical professionals use a standard set of up to three methods to date pregnancies: last menstrual period, ultrasound, and a physical exam.That way, regardless of where they trained or where they practice, any two doctors dating a pregnancy will predict the same due date or gestational age.
When people discuss how far along in pregnancy someone is, they tend to do so in general terms: “She’s in the first trimester” or “She’s four months along.” However, many people have only a vague idea of what those phrases really mean.
The second trimester extends from the end of the first until about 26 to 28 weeks from the LMP, and the third trimester from the end of the second until delivery.
Ultrasound Dating Ultrasound can be used to date pregnancies, especially when the LMP is not known (for example, pregnancy after a delivery but before a menses occurred or irregular menses without predictable ovulation).
For example, if in the first trimester the ultrasound estimate falls within one week of the LMP estimate, we still use LMP to determine due date and length of pregnancy.
If the ultrasound estimate differs by more than one week from the LMP, we would use the ultrasound estimate for the due date.