Epidemiology of preterm birth
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Janet Tucker, William McGuire
source:
http://www.amchp.org/MCH-Topics/H-N/Documents/Infant%20Mortality%202004
/bmj.preterm.birth.pdf
Preterm birth is a major challenge in perinatal health care. Most
perinatal deaths occur in preterm infants, and preterm birth is
an important risk factor for neurological impairment and
disability. Preterm birth not only affects infants and their
families—providing care for preterm infants, who may spend
several months in hospital, has increasing cost implications for
health services.
Definitions
Preterm birth is the delivery of a baby before 37 completed
weeks’ gestation. Most mortality and morbidity affects “very
preterm” infants (those born before 32 weeks’ gestation), and
especially “extremely preterm” infants (those born before 28
weeks of gestation).
Over the past 20-30 years advances in perinatal care have
improved outcomes for infants born after short gestations. The
number of weeks of completed gestation that defines whether a
birth is preterm rather than a fetal loss has become smaller. In
1992 the boundary that required registration as a preterm live
birth in the United Kingdom was lowered from 28 completed
weeks’ gestation to 24 weeks’ gestation. This boundary varies
internationally, however, from about 20 to 24 weeks. Some
classification of fetal loss, still birth, and early neonatal death for
these very short gestations may be unreliable.
Gestational age versus birth weight
Even in developed countries, there is often uncertainty and
incomplete recording of estimates of gestation. In most of the
United Kingdom data on birth weight data but not on
gestational age are collected routinely.
Although some concordance exists between the categories
of birth weight and gestational age, they are not
interchangeable. The categories for birth weight are:
x Low birth weight ( < 2500 g)
x Very low birth weight ( < 1500 g)
x Extremely low birth weight ( < 1000 g)
Only around two thirds of low birth weight infants are
preterm. Term infants may be of low birth weight because they
are “small for gestational age” or “light for date” infants. These
infants are usually defined as below the 10th centile of the
index population’s distribution of birth weights by
gestation—that is, in the lowest 10 per cent of birth weights.
Preterm infants may also be small for gestational age. They
may have neonatal problems additional to those related to
shortened gestation, particularly if they are small because of
intrauterine growth restriction.
Perinatal problems related to intrauterine growth restriction
include:
x Perinatal death
x Fetal distress
x Meconium aspiration syndrome
x Hypoglycaemia
x Polycythaemia or hyperviscosity
x Hypothermia
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Clinical review
Incidence
Over the past 20-30 years the incidence of preterm birth in
most developed countries has been about 5-7% of live births.
The incidence in the United States is higher, at about 12%.
Some evidence shows that this incidence has increased slightly
in the past few years, but the rate of birth before 32 weeks'
gestation is almost unchanged, at 1-2%.
Several factors have contributed to the overall rise in the
incidence of preterm birth. These factors include increasing
rates of multiple births, greater use of assisted reproduction
techniques, and more obstetric intervention.
Part of the apparent rise in the incidence of preterm birth,
however, may reflect changes in clinical practice. Increasingly,
ultrasonography rather than the last menstrual period date is
used to estimate gestational age. The rise in incidence may also
be caused by inconsistent classification of fetal loss, still birth,
and early neonatal death. In some countries, infants who are
born after very short gestations (less than 24 weeks) are more
likely to be categorised as live births.
With the limited provision of antenatal or perinatal care in
developing countries, there are difficulties with population
based data. Registration of births is incomplete and information
is lacking on gestational age, especially outside hospital settings.
Data that are collected tend to give only estimates of perinatal
outcomes that are specific to birth weight. These data show that
the incidence of low birth weight is much higher in developing
countries than in developed countries with good care services.
In developing countries, low birth weight is probably caused
by intrauterine growth restriction. Maternal undernutrition and
chronic infection in pregnancy are the main factors that cause
intrauterine growth restriction. Although the technical advances
in the care of preterm infants have improved outcomes in
developed countries with well resourced care services, they have
not influenced neonatal morbidity and mortality in countries
that lack basic midwifery and obstetric care. In these developing
countries, the priorities are to reduce infection associated with
delivery, identify and manage pregnancies of women who are at
risk, and provide basic neonatal resuscitation.
Causes of preterm birth
Spontaneous preterm labour and rupture of membranes
Most preterm births follow spontaneous, unexplained preterm
labour, or spontaneous preterm prelabour rupture of the
amniotic membranes. The most important predictors of
spontaneous preterm delivery are a history of preterm birth
and poor socioeconomic background of the mother.
Interaction of the many factors that contribute to the
association of preterm birth with socioeconomic status is
complex. Mothers who smoke cigarettes are twice as likely as
non-smoking mothers to deliver before 32 weeks of gestation,
although this effect does not explain all the risk associated with
social disadvantage.
Evidence from meta-analysis of randomised controlled trials
shows that antenatal smoking cessation programmes can lower
the incidence of preterm birth.Women from poorer
socioeconomic backgrounds, however, are least likely to stop
smoking in pregnancy although they are most at risk of
preterm delivery.
No studies have shown that other interventions, such as
better antenatal care, dietary advice, or increased social support
during pregnancy, improve perinatal outcomes or reduce the
social inequalities in the incidence of preterm delivery.
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The rate of preterm birth varies between ethnic groups. In
the United Kingdom, and even more markedly in the United
States, the incidence of preterm birth in black women is higher
than that in white women of similar age. The reason for this
variation is unclear because differences remain after taking into
account socioeconomic risk factors.
Multiple pregnancy and assisted reproduction
Multifetal pregnancy increases the risk of preterm delivery.
About one quarter of preterm births occur in multiple
pregnancies. Half of all twins and most triplets are born
preterm. Multiple pregnancy is more likely than singleton
pregnancy to be associated with spontaneous preterm labour
and with preterm obstetric interventions, such as induction of
labour or delivery by caesarean section.
The incidence of multiple pregnancies in developed
countries has increased over the past 20-30 years. This rise is
mainly because of the increased use of assisted reproduction
techniques, such as drugs that induce ovulation and in vitro
fertilisation. For example, the birth rate of twins in the United
States has increased by 55% since 1980. The rate of higher
order multiple births increased fourfold between 1980 and
1998, although this rate has decreased slightly over the past five
years. In some countries two embryos only are allowed to be
placed in the uterus after in vitro fertilisation to limit the
incidence of higher order pregnancy.
Singleton pregnancies that follow assisted reproduction are
at a considerable increased risk of preterm delivery, probably
because of factors such as cervical trauma, the higher incidence
of uterine problems, and possibly because of the increased risk
of infection.
Maternal and fetal complications
About 15% to 25% of preterm infants are delivered because of
maternal or fetal complications of pregnancy. The principal
causes are hypertensive disorders of pregnancy and severe
intrauterine growth restriction, which is often associated with
hypertensive disorders. The decision to deliver these infants is
informed by balancing the risks of preterm birth for the infant
against the consequence of continued pregnancy for the
mother and fetus. Over the past two decades improved
antenatal and perinatal care has increased the rate of iatrogenic
preterm delivery. During that time the incidence of still birth in
the third trimester has fallen.
Outcomes after preterm birth
Broadly, outcomes improve with increasing gestational age,
although for any given length of gestation survival varies with
birth weight. Other factors, including ethnicity and gender also
influence survival and the risk of neurological impairment.
The outcomes for preterm infants born at or after 32 weeks
of gestation are similar to those for term infants. Most serious
problems associated with preterm birth occur in the 1% to 2%
of infants who are born before 32 completed weeks' gestation,
and particularly the 0.4% of infants born before 28 weeks'
gestation. Modern perinatal care and specific interventions,
such as prophylactic antenatal steroids and exogenous
surfactants, have contributed to some improved outcomes for
very preterm infants. The overall prognosis remains poor,
however, particularly for infants who are born before 26 weeks'
gestation.
The outcome for preterm infants of multiple pregnancies
can be better than that of singleton pregnancies of the same
gestation. In term infants the situation is reversed. The
improved outcome for preterm infants of multiple pregnancies
has been attributed to closer surveillance of the mother and
