6. Gestational age

Table 1: Births by term, England and Wales Births % Pre-Term Term Post-term Live (99.5% of all births) 7 89 3 Still (0.5% of all births) 65 34 1 Source: Office for National Statistics Notes: 1. Figures may not sum to 100 due to rounding. 2. Known gestation only. Download this table Table 1: Births by term, England and Wales .xls

The table above shows the percentage of births in each gestational group. The pre-term gestation category can be broken down into 3 more gestation groups. Of the 7% of births that were pre-term, almost 5% were extremely pre-term (between 24 and 27 weeks), 11% were very pre-term (between 28 and 31 weeks) and 85% were moderately pre-term (between 32 and 36 weeks).

Stillbirths accounted for 0.5% of all births in 2013. Of the 3,241 stillbirths where gestational age was known, 65% were born pre-term. Of these, 39% were extremely pre-term, 25% were very pre-term and 36% were born moderately pre-term.

Figure 1 shows that the percentage of babies dying within the first year falls to below 10% in births from 28 weeks gestation onwards.

Figure 1: Percentage of infant deaths and number of live births by week gestation, 2013 England and Wales Source: Office for National Statistics Download this image Figure 1: Percentage of infant deaths and number of live births by week gestation, 2013 .png .xls

Figure 1 shows that in 2013, out of the total live births with known gestation, 1% occurred before 32 weeks gestation (earlier than moderately pre-term). Of these births 15% resulted in an infant death, which accounted for over half of all infant deaths. Almost all live births (99%) occurred at 32 weeks gestation or later, with 0.2% of these births resulting in an infant death. Infant deaths where the baby was born at 32 weeks gestation or later made up 47% of all infant deaths.

In 2013, 99% of births in the “under 22 weeks and birthweight less than 1,000g” category lead to an infant death. This decreased to 90% for births at 22 weeks gestation, and rapidly fell to less than half (41%) for 24 weeks gestation. By 27 weeks gestation (the end of the extremely pre-term category) the percentage of births that resulted in an infant death fell to 11%. The percentage of infant deaths was 1% for babies born at 34 weeks gestation. For each week from 38 weeks gestation onwards, the percentage of infant deaths fell to 0.1%. A fetus is considered viable at 24 weeks. Very few live births occur before this, and infant mortality rates for the few babies born this early are extremely high. For babies born in 2013, 0.1% of live births occurred at less than 24 weeks. The infant mortality rate for these babies was 853.5 deaths per 1,000 live births. The majority of these deaths (92%) occurred during the early neonatal period (the first week of life).

There has been little change in the distribution of birth by gestational age since 2006, when this time series was started. In the publication reporting the 2006 figures, post-term babies were reported in the same category as term babies (37 weeks and over). The report showed that 92% of babies were born after 37 weeks (term and post-term), this is the same value found in 2013.

Reference table 2 (228.5 Kb Excel sheet) shows that the infant mortality rate for babies born pre-term in 2013 was 21.1 deaths per 1,000 live pre-term births. The 2013 rate is lower, but not statistically significantly, than the rate for 2012 (23.6 deaths per 1,000 live births), the 2013 rate is significantly lower than the rate in 2006 (28.6 deaths per 1,000 live births). The rate of infant mortality of pre-term babies was significantly higher than the overall infant mortality rate (3.8 deaths per 1,000 live births). The infant mortality rate for babies born at term stayed the same as 2012 (1.4 deaths per 1,000 live births) and was significantly lower than the overall infant mortality rate. Babies born post-term (42 weeks and over) in 2013 had an infant mortality rate of 1.3 deaths per 1,000 live births, this is a non-significant rise from 2012 (0.9 deaths per 1,000 live births). The infant mortality rate for term and post-term births is significantly lower in 2013 (1.4 deaths per 1,000 live births) compared to the 2006 infant mortality rate (1.9 deaths per 1,000 live births). Recent research has indicated that babies born moderately pre-term (32 to 36 weeks) and early term (37 to 38 weeks) have an increased risk of ill health during childhood (Boyle et al., 2012).