Sleep changes at around the 12 week /4 months mark. It moves away from newborn passive sleep to become a more active endeavour.
For the period know as 'The Fourth Trimester' sleep for baby is like it was in the womb. That means that as long as all needs are met (food, warmth, comfort) then baby's passive state is asleep. It take a no effort, it's effectively the "default position" to be asleep.
As baby leaves the fourth trimester, sleep becomes more adult like - in that it forms in cycles of light and deep sleep with very brief awake periods between cycles. Also, and this is the ball-ache for parents- the process of getting to sleep is no longer passive. It is an active endeavour - baby, helped by parents, has to work to get to sleep. It no longer 'just happens'.
So, that explained, this is why you are getting changes now. Now is when you have to start establishing ways to get baby to go to sleep. Some methods you use are attachment based, so dependant on an adult. Other methods are more independent, so with a view to baby being able to do it without an adult (eventually).
Well established ways to sooth and settle a baby to sleep:
● Sucking is naturally soothing and calming
• the attachment way to achieve this is feeding to sleep
• the independent way to achieve this is use a dummy
● Movement
• attachment - rocking to sleep, sling naps
• independent - use bouncy chair or pram for naps where possible
● Enclosed, tightly held feeling
• attachment - holding baby to sleep.
• independent - swaddle
● Reassuring presence from parent
• attachment - Cosleeping
• independent - touch reassurance in bedside cot
So - you decide depending on your parenting style how yo best help baby learn to go to sleep. The habits you start now will be long term so while you need to accept that you do need to help baby sleep, also think about the longer term and how/when you will reduce the help you give.
I pick her up she's asleep, but if I try putting her back down she wakes and fusses. I persevere and eventually she will stay asleep but again only for about an hour and we have to do it all over again. Last night I was so exhausted I let her sleep on me eventually (me awake) and she slept for ages.
One of the central pillars to healthy sleep hygiene is that baby must - absolutely must - go to sleep where they stay asleep. So if you pick baby up and they go to sleep in your arms - that's where they need to stay (hence ending up on you in bed). If you want baby to sleep in the cot then you need to do the resetting with baby in the cot, not picked up.
What is to is:
● cosleeper or sidecar cot. Lean into the cot myself
● Dummy in, get baby sucking to sooth
● firm hand on chest for reassurance
● my head close to babies so she can feel and hear my breath on her cheek
● wait for resettle, then extract myself from the sidecar cot back to my bed.