Definition of Preterm :

Pre-term labour is defined as the presence of true labour (Regular painful uterine contractions associated with cervical effacement and dilatation) before the 37th week of pregnancy and after the age of viability of the fetus.

Eg 37 weeks and 1 day is not preterm while 36 weeks and 6 days is preterm.

In the UK the age of viability is considered to be 24 completed weeks of gestation -from the date of the last menstrual period-LMP or 22 completed weeks from the date of conception, assuming a 28 day menstrual cycle.

The correct term for the onset of labour before 24 weeks of gestation is miscarriage rather than preterm labour.

Predisposing factors/ causes of pre-term labour:

Obstetric and gynaecological causes:

i) Previous history of preterm/ premature rupture of membranes

ii) Cervical incompetence

iii) Multiple Pregnancy

iv) Polyhydramnios.

v) Antepartum Haemorrhage.

vi) History of previous intrauterine death of the fetus.

vii) Uterine abnormalities

viii) Idiopathic

ix) Previous history of elective delivery due to preterm or intrauterine growth retardation.

x) Vaginal infections

Medical causes:

i) Heart disease and other medical disorders

ii) Systemic infections

iii) Anaemia (Hb< 9.5g/dl) and high haemoglobin concentration (> 13.5 g/dl) is associated with a markedly increased risk of preterm.

Epidemiological Causes:

i)Women from the lower socioeconomic class of society

ii)Body mass index (BMI) <19

iii)Young and older age mothers.

iv)Smoking mothers.

v) Single mother (Unmarried/unsupported).

What is preterm pre-labour rupture of membranes?

This is the pre-labour rupture of the membranes with subsequent preterm labour. The important and well know complication of preterm pre-labour rupture of membranes is ascending infection.

Preterm Pre-labour rupture of membranes will be discussed separately on another post.

Incidence and epidemiology:

Approximately 7% of labours are preterm.

The incidence of preterm is much higher among women from the lower socioeconomic class of society. It is also much lower among more affluent areas.

The period called term (Between 37 and 42 completed weeks of gestation) is the period where 90% of births occur. This explains the fact that the length of a normal human pregnancy is much more variable than other mammals.

Diagnosis of preterm:

Although the only real proof of preterm labour is the progressive dilatation of the cervix, the diagnosis of preterm labour has to be made on the basis of uterine contractions. This is because it is too late to try any preventive measures once cervical dilatation is established.

Some of the problems associated with the diagnosis of preterm labour are:

i) Bouts of preterm uterine contractions experienced by numerous women and which are not associated with progressive dilatation of the cervix and preterm delivery.

ii) Preterm labour can be easily confused with acute presentations of abdominal pain such as in urinary tract infection (UTI) and pre-eclampsia.

A thorough history and examination should be undertaken in any woman suspected to have preterm labour as it is a leading cause of neonatal mortality.

If there is any evidence of ruptured membranes, digital examination should be avoided , as it increases the risk of ascending infection. The examination should be repeated 4 hours later if thee is any doubt doubt about the diagnosis.

Cardiotocography is  very useful in the diagnosis of preterm labour. It also helps to confirm fetal wellbeing by observing the fetal heart rate pattern. However one has to note that the absence of uterine contractions does not eliminate the diagnosis of preterm labour. A vaginal examination is still necessary.

A high vaginal swab at the time of the speculum examination for Gram stain, microscopy and culture (look for group B streptococcus- most dangerous). This will enable the use of appropriate antibiotics if infection develops.

The management of preterm labour will be covered in a subsequent post.

Feel free to ask any questions/comments below

References:

i) Chapter 24 pages 291-294 Preterm Labour by P.J Steer -Dewhurst’s Textbook of Obstetrics and Gynaecology for Postgraduates, Sixth Edition. Edited by D. Keith Edmonds.

ii) Chapter 18 pages 273-275 Preterm Labour -Obstetrics by Ten Teachers- Seventeenth Edition edited by Stuart Campball and Christoph Lees.

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