Why do they say the first three months are trimester 4? What should I be expecting?

The 4th trimester is the theory that the first three months of baby's life are very much an extension of his/her life in mommy's womb. Baby has spent about 9 months having every need attended to in mommy's womb and the 4th trimester allows baby to go through similar motions in order to acclimatise better to the "real world". To create a 4th trimester, the following are essential: 1) Baby stays close to mommy as much as possible; alot of contact, snuggles and love, just like in the womb. Invest in a good carrier to keep baby close whenever you can. 2) Feed baby on demand; instead of measuring ounces or watching the clock all the time, learn to recognise baby's hunger cues. 3) Recreate the mood of the womb; as babies are born without the concept of night and day because they spent their time in the womb according to your daily routine, you can help them sort their sleep habits by keeping things dark and quiet at night and bright and “noisy” during the day. http://www.becomingmamas.com/life-in-the-fourth-trimester/