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What Factors Determine Prematurity in Babies?

This article explores how the gestational age of premature babies is calculated in various ways in different settings.
Close up of a preterm baby receiving monitored supplemental oxygen in the NICU
© Universidad de Guadalajara CC BY-NC-SA 4.0

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

Illustration showing how to measure symphysis-fundal height from the foetal crown to the foetal rump

Click to expand

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)

Graph shows how the estimate of foetal gestational age increases from 14 to 40 weeks at a steady rate against increasing symphysis-fundal height measurements of 10 to 42 cm

Click to expand

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)

Numbers of preterm births in 2010 were much higher in the Sub Saharan Africa (nearly 4 million) and South Asia (just over 5 million) regions than in other regions of the world (which each have less than 2 million preterm births)

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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

Illustration shows how 50% of preterm babies born at 24 weeks gestational age survive when provided with neonatal intensive care compared to the 50% of preterm babies born at 34 weeks gestational age who survive when born in low and middle income countries.

Click to expand

How is gestational age determined in your setting?

© London School of Hygiene & Tropical Medicine CC BY-NC-SA 4.0
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