(PHILLY.COM) — Like any new parents, Heather and Riley Delaney are excited to see their babies hitting developmental milestones.

But for 15-month-old twins Abby and Erin, who were fused at the tops of their heads until June, even the tiniest hints of progress win tears and applause.

“Look at Abby holding her head up!” occupational therapist Ashley Binkowski said recently as she helped the baby sit on a mat at Children’s Hospital of Philadelphia. “That takes abdominal control on this soft surface.”

“Working on that core!” her father cheered.

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