I just read the actual article. It sounds like it's not 8 hours (coffee) or 5 hours (other caffeine sources) per 100 mg/day, but just in certain weeks. So... if you drank 300 mg/day during week 17, you would deliver at 41 weeks (3x8 hours x 7 days).
It's not a clearly written study, and they used a ton of stats that I don't quite understand. And while the journal is well read, it's not Nature Medicine or Science.
Pregnant women self-reported their caffeine intake & sources of caffeine at different times throughout pregnancy. What was interesting, is that there also seems to be an age & education effect too. The older you are or the higher degree you have, the more caffeine you consume.
The article: If you click on "provisional pdf" towards the bottom of the page you can read the entire study.