Epidemiology%20of%20preterm%20birth

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Epidemiology of preterm birth

• Stefan Johansson
• Department of Neonatology, Karolinska university
  hospital
• Department of Medical Epidemiology and
  Biostatistics, Karolinska institutet
• Stockholm, Sweden

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Preterm birth
preterm 36 weeks
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• Preterm birth - not uncommon
• extremely preterm 500 per year
• very preterm 1500
• moderately preterm 4500

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Preterm birth - part of being human?

• Preterm birth more common in humanscompared to
  other mammalian species.
• Evolutionary trend towards earlier birth?
• narrow pelvis related to bipedal gait
• large brain / head and risk of obstructed labour

Steer. BJOG 2005112, S11
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Preterm birth ? prematurity

• Preterm birth refers to the timing of birth.
• Prematurity refers to immature organ systems at
  birth.
• Most infants born in week 35-36 are mature and
  cared for in regular maternity units.
• infants born at term (37 weeks and onwards) can
  suffer from some degree of immaturity

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Rates of preterm birth varies in the world

• Sweden 6
• US 10
• Malawi 22
• Why do ratesvary?

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Increasing rates of preterm birth

• Due to factors such as
• in vitro fertilization
• higher maternal age
• Induced delivery before term
• ultrasound dating of pregnancy length
• However, among Danish low risk women the rate
  of preterm delivery has increased by 51 from
  1995 to 2004,from 3.8 to 5.7
• why??

Langhoff-Roos. BMJ 2006332937
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Preterm birth and mortality - world-wide

• One million infants born preterm die during the
  first four weeks (26 of neonatal mortality).

Lawn et al, Lancet 2005365891-900
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Gestational age and mortality - Sweden
Perinatal mortality () in Sweden 2003, by
gestational age.
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Very preterm birth and mortality - Sweden
Infant mortality () in Sweden 1992-98, by
gestational week
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Methodological problems - gestational length

• How is gestational length defined?
• last menstrual period
• early ultrasound (measuring bones, head and
  abdomen)

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Methodological problems - gestational length

• Gestational length, based on last menstrual
  period (LMP)
• true gestational length is shorter
• the rate of preterm birth is underestimated
• Gestational length, based on ultrasound
• more exact
• unavailable in most countries

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Methodological problems - preterm birth

• What is the method of assessing gestational
  length?
• Which preterm born infants are classified as
  alive / dead?
• live-born infants?
• still-births?
• Border of viability and neonatal care policies?
• Live-born infantsgt23-24 wks can be admitted to
  neonatal care.
• The policy regarding which infants to treat vary
  between countries.

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Methodological problems - preterm birth

• Rates of preterm birth and maternal and obstetric
  care
• is expected day of delivery (i.e. 40 completed
  weeks) calculated?
• are birth date recorded and gestational length
  calculated?
• are those data registrered?

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Methodological aspects of mortality

• Consistent definition of outcome
• Consensus on definitions
• Perinatal mortality stillbirths death during
  the first week
• Neonatal mortality during the first four weeks
• Infant mortality during the first year

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Methodological aspects of mortality

• How is alive determined at birth?
• How are deaths recorded?
• Mortality rates of live-born preterm infans
  underestimate the survival rate related to
  preterm birth
• still-births are generally not included

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Why preterm deliveries?
induced
spontaneous
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Induced preterm delivery

• Preeclampsia (hypertension and urinary protein
  leakege)
• delivery on maternal indication
• delivery on neonatal indication
• Other indications
• maternal diseases (diabetes, cancer, trauma etc,
  etc)
• fetal growth retardation
• multiple pregnancies
• malformations

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Spontaneous preterm delivery

• Infections (malaria)
• Preterm rupture of membranes (PROM)
• Preterm contractions
• Placental bleeding

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Risks of recurrent preterm delivery
Preterm birth, first pregnancy Risk of preterm birth, second pregnancy
lt 37 weeks 2.9 2.8-3.0
lt 35 weeks 3.6 3.4-3.9
lt 32 weeks 4.9 4.2-5.7
Ananth. AJOG 2006195643
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Risks of recurrent preterm delivery, 34-36 wks
2nd preterm delivery, spontaneous 2nd preterm delivery, induced
1st preterm delivery spontaneous 3.0 2.8-3.2 1.0 0.8-1.2
1st preterm delivery induced 0.8 0.6-1.0 5.8 5.0-6.7
Ananth. AJOG 2006195643
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Risks of recurrent preterm delivery, lt 28 wks
2nd preterm delivery, spontaneous 2nd preterm delivery, induced
1st preterm delivery spontaneous 13.2 8.8-19.8 12.6 7.0-22.7
1st preterm delivery induced 10.4 5.0-21.4 22.7 11.3-46.0
Ananth. AJOG 2006195643
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Preterm birth genes or environment?
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Risk factors of preterm birth - genes

• Genetic influence on gestational length.
• genetic factors accounted for 36 of preterm
  births
• Genetic component of pre-eclampsia
• heritability for preeclampsia was 31
• If the mother/sister had preeclampsia, the risk
  of preeclampsia for the daughter/sister is
  tripled, compared to unrelated women.

Clausson. BJOG 2000107375 Nilsson. BJOG
2004111200
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Risk factors of preterm birth - infections

• Reproductive tract infections have increased
  risk
• 20 of women with infections delivered preterm
  compared 9.5 of women without infection.
• More than 40 of preterm deliveries attributed to
  infections.
• Treatment of infections reduced the risk of
  preterm delivery
• Relative risk 0.16 0.04-0.66

French. AJOG 20061941717
bacterial vaginosis, Chlamydia, Trichomonas,
Mycoplasma, gr B strepto, Gonorrhoeae
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Risk factors of preterm birth - maternal age

• Young maternal age
• Teenage women have a doubled risk of preterm
  birth,compared to women aged 20-24
• High maternal age
• Women aged 40 are also at increased risk of
  preterm birth

Olausson. BJOG 2001108721 Cleary-Goldman,
Obstet Gynecol 2005105983
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Risk factors of preterm birth - education
Thompson. Paediatr Perinat Epidemiol. 200620182
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Risk factors of preterm birth - smoking
Kyrklund-Blomberg. AJOG 19981791051
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Risk factors of preterm birth - others

• Periodontal infections.
• Interpregnancy weight loss.
• Previous induced abortion.
• Air pollutants and ozone.

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Increasing rate of preterm birth - what to do?

• Reduce the prevalance of smoking?
• Screen for bacterial vaginos, chlamydia and other
  infections?
• Sharpen indications for induced preterm
  deliveries?
• More knowledge is needed aboutmodifiable risk
  factors.

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Conclusions

• Preterm birth is common and rates are increasing.
• Substantial mortality.
• Methodoligical issues.
• Many risk factors identified
• genes
• Infections
• maternal age
• Socioeconomic status
• smoking
• No succesful strategies to prevent preterm birth.