Faint Heartbeat at 7 Weeks Pregnant: What to Expect

Question
Hi, I am 7weeks 3days pregnant based on my LMP (Last Menstrual Period). I had a vaginal ultrasound yesterday to see if there's a heartbeat already and my doctor said it was faint and it's not what she expected. And that the size of my yolk sac is only 6weeks based on the size in the vaginal ultrasound. I was asked to take medication (dydrogesterone)for a week and continue my folic acid. Then go back for another vaginal ultrasound after a week to see if growth will be satisfactory and heartbeat will be faster. This is my second pregnancy and the first went well and so this time i got scared with the result of my vaginal ultrasound. Is there really something to worry about? And what could have caused it? Thank you in advance.

Answer
Vaginal USG is quite accurate in estimating the age of the pregnancy. But many times when calculated from the LMP, the  gestational age may be quite wrong, especially if you have been having irregular periods. Therefore at this moment I am really not 100% if everything is alright. But given the fact that a heartbeat was visualized, I think you should be ok.