What Factors Determine Prematurity in Babies?
Every year 15 million babies are born preterm – 1 in 10 of all babies born around the world (WHO 2012).
Gestational age (GA) describes the length of time the baby spends developing (gestating) inside the womb between conception and birth. GA is usually reported in weeks and the usual range of gestational age at birth is from 37 completed weeks to just less than 42 weeks.
The World Health Organization defines preterm birth as birth before 37 completed weeks gestational age, or as birth less than 259 days since the first day of the last menstrual period (LMP).
Preterm births are grouped by gestational age into three subcategories:
| Subcategory of preterm birth | Gestational age | Trimester of pregnancy |
|---|---|---|
| Extremely preterm | <28 weeks | Second trimester |
| Very preterm | 28 to <32 weeks | Second trimester |
| Moderate or late preterm | 32 to <37 completed weeks | Third trimester |
The causes of preterm birth are complex and largely unknown, although a wide range of triggers are known to contribute to preterm birth, including:
- Antepartum haemorrhage (bleeding from or into the reproductive organs before the birth of the baby)
- Cervical incompetence (structural weakness in the cervix, leading to inability to retain the foetus)
- Bacterial infection
- Inflammation
- Multiple pregnancy
Calculating gestational age
Gestational age is calculated in various ways across different settings. Each method has its own limitations and practical challenges.
Before birth
The first day of the last menstrual period (LMP) is the traditional way to calculate gestational age. However, this method has a low accuracy. It assumes that conception occurs on day 14 of a 28-day menstrual cycle but the length of the menstrual cycle varies between women. The time of ovulation also varies considerably from cycle to cycle and person to person. It can be a challenge for women to recall the exact LMP date. Using LMP tends to result in an overestimation of gestational age.
Clinical examination uses abdominal palpation (physical examination of the abdomen) and a measurement of the symphysis-fundal height (SFH) to assess the size of the foetus (see the illustration and graph below). The SFH is the distance between pubic bone and top of the pregnant uterus. This method is a useful guide when no other information is available. Size can be misleading in some cases, for example in a multiple pregnancy or if uterine fibroids are present or even a full bladder.
How to measure the symphysis-fundal height
International standard for clinical use: Estimating gestational age (weeks) from symphysis-fundal height measurement (cm)
(Adapted from Papageorghiou et al. BMJ 2016;355:bmj.i56623)
Download as an infographic (PDF)
Ultrasound estimation of gestational age is carried out in the first trimester of pregnancy (before 16 weeks of pregnancy). It measures the foetal crown-rump length (CRL) which is the longest demonstrable length of the embryo or foetus excluding the limbs. This method has high accuracy (95% confidence interval of plus or minus 6 days), and is the standard of care across high income countries. However, it is not always practical in low income settings (in particular rural areas) which often have poor access to ultrasound equipment and trained personnel and where late attendance, typically in the second trimester, is common at antenatal clinics.
The most accurate way to calculate gestational age before birth is to combine ultrasound and LMP but this requires trained personnel and equipment.
After birth
Birthweight is closely linked with gestational age but this method is not applicable across all settings, for example many infants are born with a low birthweight in South Asia. For this reason birthweight was removed as an objective measure in the definition of preterm birth by the World Health Assembly in the 1950s and is now only used alongside other information such as LMP.
Neonatal examination using a validated method such as the Ballard, Dubowitz, Parkin or Finnstorm scores (Lee et al. 2017), can establish gestational age. But this requires trained examiners, ongoing quality control for accuracy and time to conduct the assessment in routine clinical practice which make this type of method difficult to implement in a busy neonatal service.
Being born preterm has risks
Many preterm babies require specialist services to remain alive and quality care to reduce the risk of a lifetime of disability. The earlier a baby is born, the greater the risk of mortality, intensity of neonatal care required, and long term disabilities such as cerebral palsy and visual disorders.
Estimates based on data from 41 countries highlight that just over 15% of all preterm babies are born very or extremely preterm (see table below).
Table: Distribution of preterm birth by gestational age
| Subcategory of preterm birth | Gestational age | Proportion of all preterm births (<37 weeks) |
|---|---|---|
| Extremely preterm | <28 weeks | 5.2% |
| Very preterm | 28 to <32 weeks | 10.4% |
| Moderate or late preterm | 32 to <37 completed weeks | 84.8% |
Distribution of preterm births by region
(Adapted from Blencowe et al. 2012)
Preterm births are increasing in almost all countries in all subcategories. Over 60% of all preterm births occur in Sub Saharan Africa and South Asia. Neonatal services need to be available and accessible for all the categories of preterm birth. Currently, 50% of preterm babies born at 34 weeks gestational age survive in low and middle income countries (LMIC).
Across high income countries, the increase in preterm births is recognised as a public health priority. Advocacy has led to the establishment of different levels of neonatal care units – such as special care units, local neonatal care units and neonatal intensive care units (NICUs). Evidence based practice is part of ongoing quality improvement. Together these changes have increased the provision of high quality neonatal care for preterm babies and improved survival rates. 50% of preterm babies born at 24 weeks gestational age will survive with neonatal intensive care in high income countries.
In limited-resource settings, health system strategies to scale up neonatal services and improve preterm survival rates are a priority. Any expansion must also be accompanied by the introduction of appropriate interventions, embedded into the continuum of care, which improve preterm babies’ outcomes and reduce the risk of disabling complications like retinopathy of prematurity.
Provision of intensive neonatal care improves outcomes for premature babies
How is gestational age determined in your setting?
Retinopathy of Prematurity: Practical Approaches to Prevent Blindness
Retinopathy of Prematurity: Practical Approaches to Prevent Blindness
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