Let's be honest here - this is the dumbest book I've ever read. It could have been great in an 80s b-movie kind of way, but it was littered with problems.
One unavoidable issue is how dated it is. I'm sure it was fine in the early 80s, but for a modern reader it was just uncomfortable seeing the one black character referred to as "the coloured porter" in every page he showed up on. That one can't be helped, really. Similarly, there was a sexist tone in there a lot of the time. I can usually get away with it in older books but this one felt different for some reason.
There was a lot of pretty terrible grammar and some bad spelling peppered throughout the book. It also could have done with better editing - some of the pages were correctly indented and formatted but others were just huge blocks of text with no breaks.
One of the hardest things to stomach was the hideous lack of decent research, which lead to a plethora of innacuracies. Now, I realise this is bad horror and therefore subject to overblown imagination and extreme images. However... The author based a book around aborted foetuses but clearly had zero idea of what size or how well formed they would be. As someone who used to work in Pathology, I've seen more than a few and it made the whole thing ridiculous. The author had a vision in his head of large babies which were capable of opening their eyes and mouths, yet were somehow jellylike in texture. It was just so wrong that even suspension of disbelief couldn't cover it. There were similar issues with things even schoolchildren are taught - he wrote about lightning immediately following a thunderclap in a storm, despite the fact that thunder is caused by the lightning, and not the other way around. I realise that's probably petty, but so many little things like this added up over the book, and by the end I was almost wondering if the author had finished school. Here and there he threw in the medical name for a piece of equipment which no one would ever actually know or say - but he described machines in the hospital as "cancer scanners", which had me laughing out loud.
There were also several areas of the book where he contradicted himself a couple of paragraphs after writing the initial description. An Inspector opens a folder with 3 single sheets of paper in it, but the first report he pulls out of the folder goes on for four pages. A coroner confirms a cause of death as ectopic pregnancy but goes on to say there was no egg, embryo or foetus involved. Just weird, weird stuff, over and over again.
On top of all of the above, the whole thing just felt like a pro-life lecture - zombie babies kill their mothers for having them aborted. Every woman in the book is presented as promiscuous, having had so many sexual partners in the past that they can't even remember how many. Give me a break.
The age of the book obviously hasn't done it any favours, but I don't think that's what made it bad (for me), unfortunately.