How was it covered so suddenly that it was this well preserved while it was giving birth?
You mostest hugest mistake is the "while giving birth". It didn't have to be "preserved" while giving birth and I'm sure wasn't. It only had to
die while giving birth which we know happens; sometimes because of the birth itself other times just because of increased vulnerbility and others just bad luck.
Once dead there are lots of preserving process (especiailly in water). If it sinks into a low oxygen area or into a place where sediments are being laid down reasonably quickly (off a river mouth, bottom of an undersea cliff etc.) then the preservation can take place over days, weeks or even months.
It is just our good luck that this one was preserved and
found out of all that probably died while giving birth.