5 weeks or 6 weeks pregnant

Hello ladies. My friend is 6 weeks pregnant. She had her first ultrasound on 24th august. They saw a sack and doc said it looks like five weeks from the size. She had an ultrasound today on 30th of august they saw yolk sack and again they said that it looks like 5 weeks. As her last menstrual date was 7th July comes to 7 weeks pregnancy still the sack is 5 weeks measuring. What to do please help us . This is her first pregnancy