Puppydog, my uncle is an OB/GYN.
According to him (& my doctors too),
a baby born before 38 weeks is considered a pre-term baby.
And for many, the 40 week/9 month equation is found by using a 5 week month...which is what they usually do since technically 4 weeks is only 28 days and there is only 1 month with that few days.