Gestational Ages and Trimesters

1.   Gestational Age: The duration of the pregnancy, which most commonly is measured from the first day of the last menstrual period (LMP), or two weeks before the estimated date of conception.  Ultrasound often is used to “re-date” pregnancies when the measured duration of the pregnancy is found to be different than the LMP gestational dating.  Different providers use various cut-offs for re-dating and some choose always to use ultrasound dating over dating by LMP.

A common guideline is to use the ultrasound dating rather than the dating by LMP if the difference is:

•  1 week+ difference in the first trimester

•  2 weeks+ difference in the second trimester

•  3 weeks+ difference in the third trimester

Other terms, such as “duration of pregnancy” are ambiguous and may mean gestational age or may mean from the time of conception, which would be two weeks less than gestational age.  Even “gestational age” may refer either to dating by LMP or by conception, so the type of dating should be confirmed.

2.   First Trimester:  The first 12 -14 weeks of gestation.  There is no clear cut off and different organizations use different limits.  The National Abortion Federation textbook chose to use 13 6/7 weeks as the end of the first trimester.  Medi-Cal will pay “D&C” (first-trimester) rates up until 14 weeks, but will also pay “D&E” (second trimester) rates beginning at 12 weeks.  First-trimester abortions usually are completed by medical abortion (usually to 9 weeks) or aspiration curettage (often referred to as dilation and curettage, or “D&C”).

3.   Second Trimester:  From the end of the first trimester to 24, but sometimes up to 26 or 28 weeks.  Like the first trimester, there is no consensus on the gestational limits.  Second-trimester abortions usually are completed by dilation and evacuation (“D&E”) or by labor induction (also called “induction termination” or sometimes “medical abortion”).

4.   Third TrimesterFrom the end of the second trimester to birth.