I can tell you, From experience that a baby's heart is fully developed by week 18, which is why doctors do fetal echos between 18-24 weeks, it's the soonest they can visualize the heart without the bones getting in the way or "shadowing" it, and also the latest for them to detect holes and other issues.
There are many cases when a baby has a hole or two early on and as the baby grows and the heart continues to grow the holes close or get smaller on their own.
If you break embryonic and fetal development into thirds then when you are in your 28th week the baby begins its final third of development. (Gestation is 266 days or 38 weeks of development). So either way you look at it you are in your 3rd trimester!
I heard as early as 14 wks but the probability goes up if you wait longer. Do you not have a fetal photo or sneak peak ultrasound around you? They'll tell you.
Oh and try not to stress and worry of course u will a little but it will probably be for nothing...
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