QuoteIt would've been nice to know they had the capability to check for such contaminants in real-time.
I don't believe they do.
Ripley had to actively conduct a focused search using the medscan. It's not like it had picked it up after evacuating the Sulaco and there was a message waiting for her after the crash saying 'You have 1 new embryo in your chest'.
If they had used something like PUPs, then that's a different matter.