Hallow Dear,
There are various causes which can cause sudden intrauterine death of the baby. However, as you were rightly informed,
Trisomy 21 and congenital heart defect should not lead to intrauterine death of the baby.
If I put two plus two together, I would think in the following lines:
Your baby had congenital problems. There is sudden death of the baby inside the
uterus. Both these events can take place if the mother is diabetic or develops
gestational diabetes. Hence, I would advice you to undergo
glucose tolerance test to find out your diabetes status.
If you had very
high blood pressure, it may lead to abruption of the placenta leading to sudden intrauterine death of the baby.
Moreover, please get your and your husband's chromosomal study done which might throw some light on any other cause of the sudden intrauterine death of the baby.
You should take an opinion of Foetal medicine consultant.
I hope this helps you.
Dr. Nishikant Shrotri