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.
But remember -- you want to be sure you deliver a full term baby if possible. How long is full term? Pregnancy lasts for about 280 days or 40 weeks. A preterm or premature baby is delivered before 37 weeks of your pregnancy. Extremely preterm infants are born 23 through 28 weeks. Moderately preterm infants are born between 29 and 33 weeks. Late preterm infants are born between 34 and 37 weeks.