Overview
The estimation of pregnancy dates is important for the mother, who wants to know when to expect the birth of her baby, and for her health care providers, so they may choose the times at which to perform various screening tests and assessments,
such as serum screening, assessment of maturity, and induction of labor for postdate pregnancies.
The 3 basic methods used to help estimate gestational age (GA) are menstrual history, clinical examination, and ultrasonography. The first 2 are subject to considerable error and should only be used when ultrasonography facilities are not available. The date of feeling the first fetal movements (quickening) is far too unreliable to be useful. The date of the first documented positive pregnancy test and the beta-human chorionic gonadotropin (bHCG) level may help ascertain the minimum gestational age. In women who conceived following assisted reproduction techniques, the date of embryo transfer is known and may date the pregnancy accurately. In rare cases, the date of coitus is known, and this may be useful in calculating the length of pregnancy.
For related information, see Medscape’s Pregnancy Resource Center.
Terminology
Gestational age (GA) refers to the length of pregnancy after the first day of the last menstrual period (LMP) and is usually expressed in weeks and days. This is also known as menstrual age. Conceptional age (CA) is the true fetal age and refers to the length of pregnancy from the time of conception. This terminology does vary geographically and over time, and it may need clarification if not explicitly defined in relevant articles.