Baby measuring 4 weeks behind at 20 week anatomy ultrasound 😔

As

So I went in for my anatomy scan a couple days ago and found out the baby is measuring at 16 weeks. I am 20 weeks pregnant. I am certain of the dates because I had to go through a fertility specialist where I was put on clomid to help me ovulate. I have been to multiple doc appointments since conceiving so asking me if the dates are correct is out of the question!!! I know that I am 20 weeks.

Now that that is out of the way... I am wondering has this happened to anyone else? What was the outcome of being 4 - 5 weeks behind?... I am now seeing a high risk doctor who cant explain to me how this is possible either. He has referred me out to a pediatric cardiologist to check the baby's heart. Baby seems fine but is growing at an extremely slow pace. HELP!!! WHAT WAS YOUR EXPERIENCE LIKE PLEASE? IM SO WORRIED!!!