3.2 Small babies
Development continues in the womb until birth, which, in humans, is about 38 weeks after conception. (The often quoted duration of pregnancy of 40 weeks is based on pregnancy beginning on the first day of the last menstrual period.) The duration of the period of development before birth, called gestation, is highly variable. It is not possible to determine its full range in the UK, because medical intervention usually prevents pregnancies going beyond about two weeks after the due date, or term, although 10 month (43–44 week) pregnancies are not uncommon. Many babies, however, are born well before their due date. Those born more than three weeks before their due date are called pre-term. Pre-term babies clearly have a very different experience from full-term babies.
Consider two babies, 38 weeks after conception, one of them is pre-term, born say at 35 weeks. In what ways does the experience of the 38-week-old pre-term baby (born at 35 weeks) differ from that of the full-term baby?
There are many differences that could be mentioned. For three weeks, the pre-term baby has been physiologically independent; directly exposed to light, smells and air; able to stretch (no longer constrained within the womb); in direct physical contact with objects, (no longer buffered by the mother and surrounding amniotic fluid).
It is reasonable to ask whether these differences, which may last for eight weeks or more depending on how many weeks pre-term the baby is born, affect the development of the baby.
Consider the data in Table 1. The figures are means with the range of values in parentheses. In this study, full term was 40 weeks.
Table 1 Comparison between 25 pre-term and 39 full-term babies on two measures taken at birth (neonatal measures) and three measures taken in their 9th year (i.e. after their 8th birthday). (Full term was 40 weeks)
|Measure||Pre-term (N= 25)||Full-term (N= 3 9)||Probability values|
|Gestational age/weeks||28.7 (26–33)||39.4 (37–42)||p< 0.001|
|Birth weight/g||997.4 (700–1240)||3394.8 (2212–4648)||p< 0.001|
|9th year measures|
|Full-scale IQ||93.2 (49–126)||116.7 (75–145)||p< 0.0001|
*The adjusted age is measured from the expected due date, not the date of birth.
How many weeks before term were the pre-term babies born (approximately)?
About 11 weeks. The mean gestational age of the pre-term babies was 28.7 weeks which is 11.3 weeks before term, at 40 weeks.
How many weeks premature was the most pre-term baby?
The most premature baby was born at 26 weeks, so it was born 14 weeks before term.
The average IQ score in their 9th year, for those babies who were born pre-term was significantly less than the average IQ score of the similarly aged full-term babies.
Does it follow that a baby born prematurely will have a low IQ compared to a baby born at full term?
It does not follow that a baby born prematurely will have a low IQ compared to a baby born at full term.
Look at the range of values for IQ. Within this relatively small sample of 8(+)-year-olds, some who were premature babies achieved IQ scores well above the scores of some who were born at full term. (Some pre-term babies had IQs of over 120, while some full-term babies had IQs as low as 75.)
In the same study, the authors used MRI scanning to measure the sizes of various parts of the brain. Whilst the basal ganglia, amygdala and hippocampus were significantly smaller in those born pre-term, their ventricles were significantly larger.
This study illustrates an important aspect of development. Factors (prematurity in this case) that impact on development can alter both physical characters (height and brain size) and behavioural characters (IQ) for some considerable time after the impact (8+years in this case). (Prematurity is revisited in Section 4.3.) Prematurity is a rather general environmental variable involving lots of different factors. The next section considers a very specific environmental variable, light, and its impact on the development of vision and the visual system.