I had my first check-up two days ago and the doctor mentioned that the size of the gestational sac is smaller than expected for 6 weeks. After the scan, he changed his statement and said it could be a one-week delay and asked me not to worry. Additionally, he couldn't detect a heartbeat during the scan and asked me to come back for another check-up in two weeks. Is it abnormal to not hear a heartbeat at 42 days (6 weeks) of pregnancy?