Blood work was wrong?

I had my (almost) 5 month ultrasound today and it turns out Im only almost 4 months. Wouldn't the blood work have been wrong then? The baby and everything is measuring/dating at 14 almost 15 weeks. i was supposed to be almost 19 weeks. Its not a matter of a underdeveloped fetus either. He/She is perfect just not at what the doctor said.