Expectant mothers hope for a birth that is on or around their baby's due date. Even if you're sure about the conception date, the length of pregnancy can still vary by about five weeks, meaning birth and delivery are anything but predictable. Mothers begin to wonder: "What's the difference if my baby is born too early, on time, or too late?"

The answer: Brain development.

A recent study published in the journal Pediatrics found full-term babies experience more brain growth during the third trimester (final 13 to 14 weeks of pregnancy) compared to premature infants, who are born before 37 weeks.

"What this study shows us is that every day and every week of in utero development is critical," said Catherine Limperopoulos, senior author of the study and director of the Developing Brain Research Laboratory at Children's National Health System in Washington, D.C., in a statement.

Typically, during the third trimester, fetuses' brains grow exponentially, and the body shifts energy to the brain to make room for a neurological growth spurt. Therefore, as the brain increases four times in size, it develops critical structures and connections that will be relied upon for life. When compared to infants born at full term, preemies' brain growth is disturbed, but it has remained unclear when preemies' brain development begins to steer off course, and how that impairment evolves over time.

In an effort to observe this critical phase of brain development, Limperopoulous and her colleagues examined 75 preterm infants born prior to 32 gestational weeks weighing less than 3.3 pounds at birth. These preemies were matched with 130 infants born full term at an average age of 39 weeks of pregnancy weighing about 7.5 pounds. The research lab used three-dimensional magnetic resonance imaging to carefully record week-by-week development of the normal in utero fetal brains as well as week-by-week characterizations of specific regions of the fetal brain.

When comparing third trimester brain volumes, preemies showed lower volumes in the cerebrum (largest part of the brain controlling speech, thoughts, emotions, learning, and muscle function); cerebellum (helps with learning and social-behavioral functions and complex motor skills), controlling the balance to stand up and to walk; and the brain stem (acts as a router, facilitating communication between the brain, the cerebellum, and the spinal cord).

"If at all possible, we need to keep fetuses in utero to protect them from the hazards that can occur in the extra uterine environment," said Limperopoulous in the statement.

A Northwestern University study in June also suggests keeping the fetus in utero full term can lead to smarter babies. The researchers compared the birth certificates of 1.4 million children in Florida to their school records, with each child born at 37 to 41 weeks of gestation. This data was compared to data on late-term babies (born at 41 weeks) to those born at full term (39 or 40 weeks) with three school-based cognitive measures and two physical outcomes — abnormal newborn conditions and physical disabilities noted in the school record.

Late-term infants showed better cognitive function via higher test scores in school, and possessed a 2.8 percent higher probability of being gifted, and a 3.1 percent reduced probability of poor cognitive outcomes compared to those born at full term. However, late-term babies didn’t fare as well as full-term infants when it came to physical function, with higher rates of abnormal conditions at birth, respiratory disease/distress, and a 2 percent higher chance of physical disability by age five or six.

Full term babies may fare better with physical outcomes, and late-term babies may fare better with cognitive outcomes, however, preemies can experience problems all throughout their lives. For example, premature birth affects a baby’s brain by increasing the probability of long-term intellectual and developmental disabilities for babies.

"What is really unique about these study results is for the very first time we have an opportunity to better understand the ways in which we care for preemies throughout their hospitalization that optimize brain development and place more emphasis on those activities," said Limperopoulos.

Mothers-to-be should carefully rethink their decision to have an earlier term birth (if the circumstances of the pregnancy give them a choice) because it may affect either the physical or mental health of their child.

Source: Bouyssi-Kobar M, du Plessis AJ, McCarter R et al. Third Trimester Brain Growth in Preterm Infants Compared With In Utero Healthy Fetuses. Pediatrics. 2016.