Should I worry if my ultrasound showed calcium deposit on my unborn baby's heart and I'm 38 years old?

Follow, but... This needs to be followed, but i wouldn't worry. These deposits are actually found quite often as incidental findings on ultrasounds between 18-24 weeks. The vast majority of them clear on their own by birth. As pediatricians, we often get these reports of these findings on prenatal us, but the baby is just fine. At age in upper 30's though, be followed by high risk OB or maternal-fetal medicine!
See below. It is a non-specific term reflecting an area of the heart that might have increased minieralization, and so show up more on an echo. Can be found in normal hearts at a rate of about 3%. There is a "soft" association with genetic abnormalities in the child, so your OB will want to keep an eye on this or proceed with more testing.