Obstetrics and Gynecology
First trimester
A gestational sac is visible transvaginally around 4.5–5 weeks. Evaluate sac location, yolk sac, and embryo. Crown–rump length is the most accurate dating parameter in the late first trimester (about ±5–7 days) and supersedes last menstrual period when the two disagree. Cardiac activity is documented when an embryo is seen. An empty uterus with a positive pregnancy test and an adnexal mass suggests ectopic pregnancy; a pseudogestational sac is central and irregular, whereas a true sac is eccentrically placed with a decidual reaction.
Second and third trimester
Biometry includes biparietal diameter, head circumference, abdominal circumference, and femur length; AC is the best single indicator of growth disturbance. Amniotic fluid is assessed by the deepest vertical pocket or the amniotic fluid index. Placental position matters: previa suspected early often resolves by migration, so recheck after 20 weeks. Umbilical artery Doppler assesses placental resistance.
Gynecology
Ovarian follicles are anechoic and simple cysts are thin-walled; complex cysts with septations, solid components, or ascites raise concern. Endometrial thickness is measured in the sagittal plane; fibroids are typically well-defined and hypoechoic.
Trap: dating by LMP when a reliable CRL exists, or calling a low-lying placenta previa too early.
Rule of thumb: first-trimester CRL is accurate to about ±5–7 days and wins over LMP.