"In the beginning...": Determining Gestational Age and Due Date

The other day, I repeated a conversation I have had countless times. I was finishing up an early ultrasound on a very young woman and informed her that she was 8 weeks’ pregnant and congratulated her on coming in early for prenatal care. At that point her also very young boyfriend glared at me and at her and she burst into tears saying, “I can’t be 8 weeks pregnant. I only had sex 6 weeks ago and that was the first time in my life.” “That’s perfect,” I replied, “In obstetrics, the way we figure things, you are considered to be two weeks pregnant the day you conceived, so you probably got pregnant just when you thought.”

From the looks of disbelief and mistrust, I knew they didn’t understand, so I went on to explain. When women have regular menstrual cycles every 28 days, they usually ovulate (hatch an egg)around day 14 (counting from the first day of the last period). Once they ovulate, they have only about 24 hours that the egg can be fertilized and fertilization actually occurs in the fallopian tube. Over the next 3-4 days, the fertilized egg divides several times as it travels down the tube to an implantation site within the uterus. Since most women have fairly regular menstrual cycles, and long before we knew when conception actually occurs, the tradition was begun that the due date was calculated from the beginning of the last period. "Thus, as I was saying, you are two weeks’ pregnant the day you got pregnant. It’s a tradition we have continued for convenience and for the confusion of our patients."

From the first day of the last period in women who have regular periods, the due date is approximately 280 days, corresponding to 40 weeks. This can be estimated by counting 9 calendar months from the first day of the period and adding 7 days. Alternatively, you can pick up a pregnancy “wheel” or use an online “calculator.” Today, estimates of gestational age by ultrasound are generally accurate within 3-5 days in the first trimester of pregnancy, within 10-14 days in second trimester, and become much less reliable (no better than +/- 2-3 weeks) after that. Prior to ultrasound, we confirmed gestational age by the time of "quickening" (the first perception of fetal movement by the mother) and the first detection of the fetal heart tones by a special stethoscope (a fetoscope), both of which generally occur at about 18-20 weeks. Pregnancy dating is important because we use it to schedule certain laboratory studies (e.g., serum screening for neural tube defects and chromosomal abnormalities; screening for diabetes) that can only be interpreted reliably if the “dates” are known. We also depend on accurate dating to schedule fetal testing in women at risk for complications and for elective deliveries by cesarean section or induction of labor.

“So,” I said, “we now know how pregnant you are and as a result we now have a firm due date that will not change, even if the baby is bigger or smaller on follow-up studies.” Her only response was “Doctor, so how many months does that make me?” I could only sigh...."Lunar months or calendar months?" I asked.

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