Xtro-3: Watch the Skies
1995 Sci-fi horror movie: wikipedia imdb
It fits your description perfectly:
From 1995 so you could've watched it in the late 90ies. Alien couple caught by scientists and the fetus of the female alien was removed. Male alien is angry and kills the scientists. It's then trapped in a concrete block surrounded by forest. When the concrete block gets a crack, the alien escapes and kills people. Also blood and pieces of skin due to the way the alien kills humans. The end reveals it to be a flashback type of movie with the survivor telling the story.
- My own recollection (I watched it a few years ago)
Two aliens crash landed on Earth and were captured by scientists.
They conducted a vivisection on one of the aliens (can very well have been a pregnant specimen) and kill it. The other alien is enraged and frees itself from the cage killing the researchers. That whole scene was in black and white.
To prevent further carnage the alien is encased in a concrete block. It's on a remote island surrounded by forest. Many many years later a team of soldiers is sent there on a different mission. For sure, they are there because of the alien without knowing it. The concrete block has a crack (I'm not sure if it was already there or if the soldiers caused it). The alien gets free and is still looking for revenge. It brutally murders one soldier after another the way the other alien was killed (dissecting them while they're still alive).
Not part of your description:
The alien can camouflage itself by becoming invisible and produce some goo to trap people in. The masterminds of the operation who are after the alien also come to the island. In the end, the last surviving mastermind is in a pool of water and outside are the last surviving soldier and the alien. The soldier knows now that the mastermind is the evil one and when he is told to shoot it (the alien), he says yes but kills the mastermind instead. I think the alien then leaves Earth.
- Question linked to in a comment with X-Tro 3 as the accepted answer:
Then the footage shows scientists cutting the female alien and removing a fetus from her womb.
- IMDb review by ApolloBoy109:
Confirms the concrete block and also the last bullet point:
The flash back sequence, the humorous filmreel at the start of the film and the story within a story worked well.
- The full movie (I'm not sure I'm allowed to link to it but it's on youtube: search for "Xtro 3 Watch The Skies 1995 with bg subs"):
It's 1:36:41 long and at the 1:30:00 mark you see the flashback scene with the survivor (played by Sal Landi) telling the story to a journalist. (At the 56:00 mark you also see the scientists remove the alien fetus)