What is gestational age?
Gestational age is the number of weeks that have passed since a woman’s last menstrual cycle began. We define babies born before 37 weeks gestation as premature. A baby or fetus is considered viable from 24 weeks gestation; births between 24 and 36 weeks gestation are defined as pre-term. Babies born between 37 and 41 weeks gestation are considered to be full-term births.
More premature babies born to women aged 40 years or over
Women who gave birth in 2011 in England and Wales were more likely to give birth prematurely if they were aged 40 years or over; 10.1% of babies born to mothers aged 40 years or over were born before 37 weeks gestation, while mothers aged 25 to 29 years were least likely to give birth prematurely (6.7%). This has been a consistent trend over time. In 2007, 9.5% of babies born before 37 weeks were to mothers aged 40 years or over compared with 6.9% and 7.0% for mothers aged 20 to 24 years and 25-29 years respectively.
The link between premature birth and multiplicity
Part of the reason for the higher percentage of older mothers giving birth prematurely may be because they are more likely to give birth to multiples. In 2011, 6.2% of babies born to mothers aged 40 years or over were multiples compared with just 1.2% of babies born to mothers in their teens. For mothers of all ages the percentage of multiple births was 3.2%.
Singleton and multiple live births have very different gestational age distributions. In 2011, only 5.6% of singleton births were born pre-term, compared with more than half (53.1%) of multiple births. Most women with a twin pregnancy give birth earlier than women with a singleton pregnancy, at around 36 to 37 weeks gestation.
For all babies born before 37 weeks in 2011, the percentage of singletons born to the oldest and youngest mothers was similar (6.8% and 6.9% respectively). However, the percentage of multiple births born before 37 weeks was greater for the oldest mothers: 3.3% compared with 0.9% for the youngest mothers.
What is small for gestational age?
Low birthweight is known to be a contributing factor for poor birth outcomes. Birth weight percentiles at given gestational weeks are used to define whether or not a baby or fetus is small or large for its gestational age. A baby considered small for gestational age (SGA) will typically weigh less than the 10th percentile for babies of the same gestational age; this means that they are smaller than 90% of babies of the same gestational age. For example, a baby born weighing 2,000 grams at 37 weeks would be SGA but this would be a normal weight for a baby born at 33 weeks.
Younger mothers more likely to have smaller babies
In 2011, babies born in England and Wales were more likely to be SGA if their mother was under 20 years of age; 11.3% of babies born to mothers under 20 were SGA, compared with 8.2% of babies born to mothers aged 40 years or over, and 7.5% of babies born to mothers in their thirties. Although these figures have fallen since 2007, when 13.3% and 8.7% of babies were born SGA to mothers under 20 and 40 years or over respectively, the pattern has remained the same.
A number of studies have found that young mothers are more likely to drink and/or smoke and are less likely to have completed higher education. Socio-demographic factors such as this could play a significant role in explaining the link between maternal age and the risk of giving birth to an SGA baby (Bakker et al, 2011; Raisanen et al, 2013)
Contribution of socioeconomic status to the risk of small for gestational age infants – a population-based study of 1,390,165 singleton live births in Finland (Raisanen S, Gissler M, Sankilampi U, Saari J, Kramer MR, Heinonen S, International Journal for equity in Health May 2013).
Explaining differences in birth outcomes in relation to maternal age: the Generation R Study (Bakker R, Steegers EAP, Biharie AA, Mackenbach JP, Hofman A, Jaddoe VWV, BJOG: An International Journal of Obstetrics and Gynaecology, Volume 118, Issue 4, pages 500-509, March 2011).
These statistics were compiled and analysed by the Mortality Analysis team in the Life Events and Population Sources Division. If you’d like to find out more about Infant Mortality you can read our series of Gestation-specific , Unexplained Deaths in Infancy and Child Mortality bulletins. If you have any comments or suggestions, we’d like to hear them! Please email us at: CIM@ons.gsi.gov.uk