This is a question for your doctor. There are several reasons why embryos suddenly stop growing.
The most common is that they never really begin to grow, which is called a "blighted ovum". The egg gets fertilized and some time around when it first begins to grow, it fails to grow and then an early miscarriage happens.
It sounds like in your case, twice you had a baby with a beating heart? If that's the case, your doctor would be better able to tell you why than an internet forum.
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