Ultrasounds in Later Pregnancy Between 18 and 28 weeks of gestation, the margin of error increases to plus or minus two weeks. After 28 weeks, the ultrasound may be off by three weeks or more in predicting a due date.

What are the signs of an abnormal pregnancy?

Top 5 Conditions of Abnormal PregnancyVaginal bleeding during pregnancy. Abdominal discomfort, cramping or pain. Frequent headaches and blurred vision. Excessive thirst and sweating. No fetal movement or reduced fetal movement at more than 20 weeks gestation.