Soft spot slightly sunk in

Lauren • Had my sweet girl 02/22/20 - 5 weeks early due to rare genetic disorder💗 Had my handsome man 09/30/22 - 3 weeks early due to low birth weight💙

My daughters soft spot is a little sunk in. She’s 7 weeks. The dr said if it ever happened, it means she’s dehydrated. How do I keep her more hydrated?

She’s still peeing every diaper and acting the same, but she’s been sleeping longer (4-5 hour stretches) and I didnt want to wake her up to feed her. Could that be why?