Being able to pick up a cat "baby in arms posture" is rare. This comes from absolute trust, relaxation on the cats part, and training when they're very young, lots of attention. They do this because they enjoy it, and it really is an act of submission, because they're not as alert in that position.
My cat Keylah was dumped in my yard when she was 6 weeks old. She was tiny, she was a cold, scared little fluff ball. I cornered her, brought her inside, and she was constantly in my room, on my bed, in my arms sleeping with me. During the day, I'd have her snuggled between my boobs and just under my chin. She'd stretch out and she'd relax. To her, it was a comfort thing, and it formed the closest possible bond between us.
To this day, Keylah who is now 5 years old, will often want "baby in arms posture" with me. When she comes up and sleeps in my arms, she doesn't curl up on the bed, she will stretch out, rest her head on my arm and under my chin, I'll pull the blanket up over her and we could fall asleep like that.
I've got a lot of other cats, all of whom I'm close to, but whom have different personalities. Most of whom I've raised from newborns. They all love me to pieces, but if I try this position, they panic. Yes they trust me, but they don't like it.
This was made harder recently, because one of my cats had a baby girl Shy`el, and Shy`el shaped up to be like keylah. She curled up in our arms, she stretched out, she would bound over to cuddle us, she just loved it, under our chins, she was a tiny fluff ball doll.
Then one day I came home, and she was near death, she was so young (7 weeks) and what I thought were just sniffles manifested into a secondary infection in her lungs, and she passed 4 days later in the vet clinic I had rushed her to.
This crushed me, because when a cat reacts like you want, you form the closest bond with them so quickly, you love them like your child and it's devastating when you loose them.
It really depends on the individual cat, the breed & how you raise them, and if you ever have one that reacts like this - count yourself lucky, because this is their way of telling you how much they truly love you, rely on you, & trust you, it's one of the most amazing feelings in the world.