A preterm birth is a birth occurring before the normal end or term of pregnancy. The normal duration of pregnancy is 40 to 41 weeks. Nonetheless, there are degrees of preterm birth; these are defined according to recommendations of the World Health Organization (WHO).
Gestational age is the criterion used to define preterm birth. Any child born before 37 completed weeks of gestation, or 8 months of pregnancy, is considered to be preterm or premature. Very preterm births are those that take place from the 5th through the 7th month of pregnancy.
The information below reflects the state of knowledge in 2010, prior the launch of the Epipage2 cohort study.
In industrialized countries, between 5% and 12% of births occur before the normal term of pregnancy. In France, there are 60 000 each year, 10 000 (1.2%) before 32 weeks of gestation, and their numbers have been growing regularly. One of every 5 preterm babies is very premature.
The birth rate in France has been rising since 2000: 775 000 births in 2000, 778 000 in 2001, reaching 807 000 in 2008, for an increase in the birthrate of more than 9%. During the same period, the number of children born of multiple pregnancies has grown from approximately 21 000 in 1996-1997 to 33 000 in 2001 ; they now account for more than 4% of the births. These two factors — the increase in both births and multiple pregnancies — have led to a substantial increase in the number of preterm births.
What Are the Causes of Preterm Birth ?
There are two types of preterm birth — induced and spontaneoux.
Induced or iatrogenic preterm birth is induced by physicians who, in the presence of factors potentially threatening the mother's or child's survival, decide to end the pregnancy by inducing labor or by performing a cesarean. For example, the mother's life may be in danger because of preeclampsia (gestational hypertension related to the placenta), or the fetus's life may be threatened by the slowing of fetal growth.
During a so-colled spontaneous preterm birth, labor begins before the pregnancy has reached a normal term. We do not know the reasons for preterm labor, but it may be associated with an infectious environment. Premature rupture of the membranes (amniotic sac) can also cause spontaneous preterm birth.
On average, 60% of preterm deliveries are spontaneous, and 40% induced by physicians. The causes and conditions reponsible for these two large groups of preterm births are very different. The causes of preterm birth remain largely unknown, especially beacause they are multiple. They probably result from maternal or fetal complications and from placental abnormalities.
Data collected in the Epipage 2 study will enable us to establish with precision the respective roles of theses different causes so that we can better characterize preterm births.
What are the consequences of preterm birth ?
In the short term
The neonatal mortality (death of children in the first month of life) rate exceeds 10% for the very preterm infants (<32 weeks), and is 2-3% for those moderately preterm (32-33 weeks), and 0.5-1% for those born at 34-36 weeks, compared with less than 2 per 1000 children born at term.
In the longer term
The risks described most often are motor disabilities and intellectual impairment. As the table below shows, the earlier the preterm birth, the higher the risk.
|Motor disabilities||Intellectual impairment|
|Very perterm birth (<32 weeks)||10%||15%|
|Moderate preterm birth (32-33 weeks)||4%||10%|
|Late preterm birth (34-36 weeks)||0,5%||5%|