Value and Limitations of Obstetrical Ultrasound: Uncovering abnormalities at earlier stages.

First-trimester ultrasound examination is of value to establish dates or assess fetal viability in cases of bleeding but provides limited information on the well-being of the fetus. At between 16 and 20 weeks' gestation, examination of fetal anatomy allows many abnormalities to be detected. Third-trimester ultrasound examination is predominantly for follow up of growth and normal development or of any abnormalities seen earlier.