Baby 14 days behind but still growing

I'm pregnant via IVF but baby's growth is slow. Yesterday’s ultrasound showed measuring at 6wks.and 3days (with no heartbeat), but I should be 8 wks and 3days (baby is 2 wks behind). The ultrasound I did 8 days ago measured 5 weeks and 5 days (so baby grew 5 days worth over 8 days) no heartbeat back then either.

I'm currently 40 and have been successful getting pregnant several times through IUI and IVF over the last 2 years, but in the first trimester (up to 7 wks) I miscarry.

Did any of you experience slow growth after IVF early on and growth catch up with heart beat. Or is this bound to be another miscarriage?