What's in a name - why are we called Four Trimesters?

With only 25% of our adult brain size, we are the least mature at birth of any mammal. Anthropologist, Ashley Montagu concluded that given our upright position and large brains, human infants are born prematurely while our heads can still fit through the birth canal, and that brain development must therefore extend into postnatal life. He believed the human gestation period to actually be eighteen months long - nine in the womb and another nine outside it, and that touch is absolutely vital to this time of "exterogestation." - Cori Young

A pregnancy consists of three trimesters but the early stages of a baby's life outside the womb are often referred to as the 'Fourth Trimester' of childbearing.

The Fourth Trimester is a period when your new baby is developing rapidly. This period is typically said to last between 3 months to one year. It is especially important to human beings for developmental reasons.

There are two typical growth patterns in mammals. Some mammals (called altrical mammals) have multiple small, underdeveloped young; often in largish litters. The brains of the new babies are typically half the size of the adult. Nurturing these infants demands much time and attention, which is why it is they are typically born in broods, to allow multiple offspring to be raised at one time. A litter of new born puppies is an example.

The other growth pattern for mammals is the precocial one. The young of this kind are born as individuals and are fully functioning from the time of birth. Examples include whales and horses.The brains of all mammals, except humans, grow rapidly during gestation (pregnancy) but then slow down considerably from birth. Humans, however, maintain a high brain growth rate, tripling their brain's weight from birth to adulthood. Even though humans show precocial traits, we maintain rapid brain development. This allows humans to have a shorter gestation than we would otherwise, which allows for the baby to be born at a relatively small size and hence be able to fit through the mother's pelvis.

Understanding this biology underscores the great importance to baby humans of the "fourth trimester" - the first 3-12 months after birth. As the baby's brain develops rapidly and grows considerably, it needs a huge amount of emotional and physical nurturing, which is provided by constant, direct and immediate physical contact with the primary caregiver, breastfeeding, co-sleeping, and tuning into the baby's needs.

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