Not as far along as thought

Well, according to my lmp I should be 10wks6days today. I had my first ultrasound today and the baby is measuring 8wk3days. The heart rate was 176. My doctor says it's because my period was irregular before conception. But my period was regularly irregular, if that makes sense. It came every month, just a week or so later than normal. Does anyone have insight to this? I'm so worried something might be wrong with the baby and isn't growing right. Anyone else measure smaller than anticipated and everything turned out fine? Thanks y'all.