This was at Terminal 2 (I think) and the flight arrived late at night. So it took me 25 minutes (to the car park not the train station), please remember this is my experience and it could take longer for you. You have to take into account: flight delays, baggage delays, queues, crowds, getting lost etc.
the first trimester is from week 1 to the end of week 12 the second trimester is from week 13 to the end of week 26 the third trimester is from week 27 to the end of the pregnancy source: http://kidshealth.org/parent/pregnancy_calendar/pregnancy_calendar_intro.html.
Answer: We talk a good game about having nine months of pregnancy. But if you look at it mathematically, there are an average of 40 weeks in a normal pregnancy. If you calculate a month as being four weeks, that makes 10 months of pregnancy in every 40 weeks. The problem with this calculation is that it has 28 day months, which is not the norm in our calendar months. There is also the issue of when you actually are pregnant.