Fetal Growth by Trimester

Gestational age is the ruler used for every fetal weight, milestone, percentile and viability decision. If the ruler is wrong, a normally growing fetus can be labelled too small, or a growth-restricted fetus can appear deceptively appropriate. Pregnancy is conventionally dated from the first day of the last menstrual period (LMP). By that convention, the first trimester is approximately weeks 1 to 13 or 14, the second is approximately weeks 14 to 27 or 28, and the third begins around week 28. Fertilisation usually occurs about 2 weeks after LMP dating begins. The embryonic period spans roughly the third to eighth week after conception, or approximately 5 to 10 weeks by LMP dating. From about the ninth or tenth week of development onward, the conceptus is termed a fetus. When dates are uncertain, first-trimester ultrasound is preferred. An ultrasound between 7 and 14 weeks, ideally between 7 and 12 weeks, provides the most reliable dating. Crown-rump length is the most accurate parameter and is used up to 84 mm; beyond 84 mm, biparietal diameter is used. A later scan should...