Measuring Behind

After having MC I’m finally pregnant and further along than last time but everyday I am scared.

According to the last day of my period I should be exactly 9 weeks and 4 days ( I know it’s never exact )

When I went to the doctor before today I was

Measuring 1 day ahead ( 6 weeks and 1 day ) when I went back the next week I was 7 weeks and 1 day ( still measuring 1 day ahead ) which one mean today I would be 9 weeks and 5 days but when I went today I was like 9 weeks and 4 days

I know it is only a day and the doctor said I am right on schedule however after a MC and with this pregnancy I had a vanishing twin I am terrified. Does anyone have experience in this ?