To calculate your baby's leaps, you must use the due date (based on the 40 weeks) and not the date of birth. Also with preemies and twins.
This is because the development of the brain starts at the moment of conception. And where the period between the conception and the birth can be different for all babies, the moment between the conception and the due date is always the same. So by using the due date it doesn't matter if a baby is born early or late. Because the brain development goes by the same rate, whether your baby is still in the womb or out in the world. You cannot expect a premature baby's brain to develop at a faster speed and therefore 'catch up'.
By using the due date, we can tell you when your baby will make a leap with a margin of two weeks either side.