is this possible?

If you didn't have a period inbetween a miscarriage and a new pregnancy... how does the doctor date the pregnancy? Just by the U/S. 
Miscarried on April 11th. No period. Ovulated on May 10th-ish. DTD on May 9th & 10th. Super super faint Positive on the 20th... U/S today said 5 weeks 6 days. I was guessing 6.5-7ish weeks. Is this possible!??