Dating of pregnancy by trimesters: a review and reappraisal.

Many patients and obstetricians divide the events of human pregnancy into three intervals traditionally termed "trimesters." This system presumably arose from an equal division of the "9 months of pregnancy" into 3-month intervals. There are several problems with this system that follows pregnancy by months or trimesters. First, the average human pregnancy lasting 280 days or 40 weeks is not evenly divisible by three, leaving one to wonder how long each trimester is. Second, conversion from "weeks pregnant" to "months pregnant" is often an estimate that can foster misunderstanding between the patient and her obstetrician. Last, following pregnancy by the Gregorian calendar does not reflect embryonic or fetal developmental milestones. We propose a revision of this system to one in which natural embryonic and fetal developmental landmarks are used instead of trimesters to define the progressive stages of pregnancy. These landmarks occur approximately at 5-week intervals allowing a more simple division of pregnancy into four 10-week quartiles, each with two 5-week intervals. This article reviews many of these important landmarks within this framework. This system emphasizes a developmentally based way of understanding the events of pregnancy for both the patient and the obstetrician.