First scan at week 7

i had my scan at my supposedly week 6 based on last period (and i have a very regular cycle of 28 days and almost always regular ovulation in mid cycle), and basically saw nothing. no sac no yolk no fetus. got a huge scare but blood test showed hcg and positive early pregnancy. My gynae suggested my ovulation could have come like 1 to 2 weeks late and late conception, so measure from last period is not always accurate. we scanned again 2 weeks+ later and this time saw the baby and its heartbeat and measured about 6 weeks 5d - 7 weeks~