The 4 week month is based on the lunar month - it take 4 weeks for the sliver of the new moon to become a full moon and back to a sliver. A 40 week pregnancy is 10 lunar months. At 20 weeks, you've completed 1/2 of your pregnancy - unless your baby is early or late.
The calendar month is longer and irregular, from 28 to 31 days, so is less precise and more confusing.
My doctor told me that most babies come within 5 days of their due dates.