Jesse McCartney says he's looking forward to becoming a father again ... but says his first son's complicated birth left him with some PTSD.

We got the singer Thursday at LAX, and he told our photog he's thrilled to be having a baby girl.

Jesse explained that the scary memories of his son's dramatic birth have put a damper on the baby prep ... telling us they're trying to stay "cautiously optimistic" that it'll be smooth sailing this time around.

As you know, Jesse's son spent 70 days in the neonatal intensive care unit after being born 3 months premature ... a traumatic experience for any parent.

Jesse says that -- so far -- everything's going great and his wife, Katie Peterson, is doing well.

Here's hoping baby No. 2 is born healthy ... ditto for mama.