We sleep trained at 5 months and my MIL still isn't speaking to me about it
Okay saying this here because I can't say it in the family WhatsApp group without getting a lecture. We did a gentle version of sleep training at 5 months — not full cry it out, but we stopped rocking her fully to sleep every single time and started putting her down drowsy-but-awake, checking in every few minutes if she cried instead of picking up immediately.
First night was rough, maybe 20 minutes of crying with check-ins. By night 3 it was down to 5 minutes and by night 5 she was mostly self-settling. She was already eating well and gaining weight fine, our pediatrician had cleared it, so this wasn't about food.
My mother in law thinks we are "hardening our own child's heart" and keeps bringing up how she never did this with my husband. I get it, it feels harsh from outside. But nobody in that generation was also averaging 3 hours of broken sleep for 5 months straight either.
Some actual useful bits if anyone's attempting this around the same age: watch your wake windows (ours was roughly 2 to 2.5 hours by 5 months, overtired babies fight sleep way harder), keep the bedtime routine identical every night so the cues are predictable, and don't start this during a leap or right after vaccinations, timing matters more than method. Also it is genuinely not for everyone and that's fine too, just tired of the guilt-tripping when it is working for us.