Born @ 35 weeks constipation ?

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So any other mamas out there had a 35 week old baby. My baby passed the black stool at hospital but he is now a week old and still hasn’t done a normal poop he is formula fed he does try to push I held his legs once and he pooped it was like clay texture. He is a little gassy and his belly is soft has anyone seen constipation this early? Or should I give it more time before switching formulas and which formula do you guys recommend for pooping . How long did it take for your premie to have normal poop?