Ultrasounds dating from lmp conception
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.Instead, we most commonly date pregnancies from the first day of the last menstrual period (LMP).One reason for not dating pregnancies from the day of conception is that we cannot know that day exactly (excluding cases of assisted reproduction), but we can know the first day of LMP, based on what a patient reports to us.This method continues in the subsequent trimesters.In the second trimester, the determination also defaults to the LMP unless the ultrasound estimate is more than two weeks different from the LMP.Medical professionals consider delivery premature when it occurs before 36 weeks, and extremely premature when it occurs before 28 weeks’ LMP.
The first trimester is generally considered to comprise the time up to 12 or 14 weeks from the LMP.
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).
It is not used by medical professionals to describe pregnancy length and is solely a political term.
Sometimes we do use a trimester system if we are seeking to describe how far along a woman is in general terms.