I remember when it came out in 1974 !
I was too young to go see in the theater in 1974(12 years old) BUT I had seen the trailer on TV and it was the scariest thing I had EVER SEEN !! It was VERY similar to those fake 70's trailer spoofed in Tarantino's GRINDHOUSE (especially the one with the low male voice saying : - If you're thinking of seeing this movie alone ... DON'T !) with the announcer saying the infamous tag line : - There's only one thing wrong with the Davis baby ... IT'S ALIVE !! With a zoom-in on John Ryan's face and the creepy sounds of the baby !! Man !
You gotta remember folks : this was before JAWS, HALLOWEEN, NIGHTMARE ON ELM STREET and other iconic (and bigger budgeted) horror films (though it did come out a few months after the granddaddy of all horror films : THE EXORCIST), and it made quite a splash back in its days (and A LOT of money for independent director Larry Cohen).