On average, pregnancy lasts 40 weeks. ACOG has defined new terms lengths as: early term is between 37 weeks and 38 weeks 6 days; full term is between 39 weeks and 40 weeks 6 days; late term is the 41st week; and post term, is defined as after 42 weeks.
ACOG's previous defintions of preterm was before 37 weeks of pregnancy, and full term was anytime from 37 to 42 weeks.
Why the change in defintion of the length of pregnancy?
In the past decade, it has been shown that weeks matter in the healthy development of a baby. There has been a trend towards early induction and early scheduled cesarian section in the past two decades, which has been shown by studies to be detrimental to baby's health.
Ana Tita, M.D. ran a research study in 2009 showing that more than 36% of all plannned cesarian sections are performed before 39 weeks gestation. The rate varied widely from hospital to hospital, indicating that the surgical births were being influenced by institution policy and scheduling issues. Another factor influencing early planned cessarain section is that women are asking to schedule the surgery earlier in their pregnancy to fit their busy schedules and also women often want to use specifically their doctor, who may only be available at certain times for surgery.
By looking at the retrospective data from 24,000 repeat births by cesarean section, the researchers found the last few weeks of gestation seriously impact infant health. The last few weeks are necessary for babies to fully develop their essential organs: brain, lungs and heart.
The study found that the last few weeks of gestation influences the baby's development and mortality in the following ways:
Babies born by cesarian section in the 37th - 38th week had a higher incidence of respiratory problmes and serious infection than babies born in the 39th week.
Babies born in their 38th and 39th week actually had double the risk of birth complications and babies born in their 37th - 38th week had four times the risk of birth complications.
Based on the past decade of research regarding baby's health, the March of Dimes launched a campaign called Healthy Babies Are Worth the Wait, which, through education, has heavily influenced consumer and professional thought about the value of waiting for full term gestation. .