Only February contains exactly four weeks (28 days), and only in non-leap years. The average month 4.345 weeks: 365 / 7 = 52.142 / 12 = 4.345 Therefore: 10

Only February contains exactly four weeks (28 days), and only in non-leap years. The average month 4.345 weeks: 365 / 7 = 52.142 / 12 = 4.345 Therefore: 10 months * 4.345 = 43.45 weeks 9 months * 4.345 = 39.105 week Neither is 40 weeks. And anyway, 40 weeks is only an approximation as it is common for births to occur from 37 to 42 weeks from the first day of the woman's last menstrual period.