title: Preterm birth and future risk of maternal cardiovascular disease - is the association independent of smoking during pregnancy?
abstract: BACKGROUND
  While the association of preterm birth and the risk of maternal cardiovascular disease (CVD) has been well-documented, most studies were limited by the inability to account for smoking during pregnancy - an important risk factor for both preterm birth and CVD. This study aimed to determine whether the increased future risk of maternal cardiovascular disease (CVD) associated with preterm birth is independent of maternal smoking during pregnancy.
  
  
  METHODS
  A population-based record linkage study of 797,056 women who delivered a singleton infant between 1994 and 2011 in New South Wales, Australia was conducted. Birth records were linked to the mothers' subsequent hospitaliation or death from CVD. Preterm births were categorised as late (35-36 weeks), moderate (33-34 weeks), or extreme (≤32 weeks); and as spontaneous or indicated. Cox proportional hazard regression was used to estimate adjusted hazard ratios (AHR) [95 % CI].
  
  
  RESULTS
  During the study period, 59,563 women (7.5 %) had at least one preterm birth. After adjustment for CVD risk factors other than smoking, AHR [95 % CI] of CVD among women who ever had a preterm birth was 1.78 [1.61-1.96]. Associations were greater for extreme (AHR = 1.98 [1.63-2.42]) and moderate (AHR = 2.06 [1.69-2.51]) than late preterm birth (AHR = 1.63 [1.44-1.85]), for indicated (AHR = 2.04 [1.75-2.38]) than spontaneous preterm birth (AHR = 1.65 [1.47-1.86]), and for having ≥ two (AHR = 2.29[1.75-2.99]) than having one preterm birth (AHR = 1.73[1.57-1.92]). A further adjustment for maternal smoking attenuated, but did not eliminate, the associations. Smoking during pregnancy was also independently associated with maternal CVD risks, with associations being stronger for mothers who smoked during last pregnancy (AHR = 2.07 [1.93-2.23]) than mothers who smoked during a prior pregnancy (AHR = 1.64 [1.41-1.90]).
  
  
  CONCLUSIONS
  Associations of preterm birth and maternal CVD risk are independent of maternal smoking during pregnancy. This underscores the importance of smoking cessation in reducing CVD and suggests that a history of preterm delivery (especially if severe, indicated or recurrent) identifies women who could be targeted for CVD screening and preventative therapies.
authors.affiliation: Clinical and Population Perinatal Health Research, Kolling Institute, University of Sydney at Royal North Shore Hospital, St Leonards, New South Wales, NSW 2065, Australia. ducanhnguyet@yahoo.com.
authors.forename: Anh D
authors.initials: AD
authors.lastname: Ngo
authors.affiliation: Clinical and Population Perinatal Health Research, Kolling Institute, University of Sydney at Royal North Shore Hospital, St Leonards, New South Wales, NSW 2065, Australia. Jscheng@med.usyd.edu.au.
authors.forename: Jian Sheng
authors.initials: JS
authors.lastname: Chen
authors.affiliation: Department of Cardiology, Royal North Shore Hospital, St Leonards, New South Wales, NSW 2065, Australia. Gemma.Figtree@sydney.edu.au.
authors.forename: Gemma
authors.initials: G
authors.lastname: Figtree
authors.affiliation: Clinical and Population Perinatal Health Research, Kolling Institute, University of Sydney at Royal North Shore Hospital, St Leonards, New South Wales, NSW 2065, Australia. Jonathan.Morris@sydney.edu.au.
authors.forename: Jonathan M
authors.initials: JM
authors.lastname: Morris
authors.affiliation: Clinical and Population Perinatal Health Research, Kolling Institute, University of Sydney at Royal North Shore Hospital, St Leonards, New South Wales, NSW 2065, Australia. Christine.Roberts@sydney.edu.au.
authors.forename: Christine L
authors.initials: CL
authors.lastname: Roberts
mesh_terms: D000328:Adult; D002318:Cardiovascular Diseases / Q000453:epidemiology* / Q000209:etiology; D003430:Cross-Sectional Studies; D005260:Female; D005865:Gestational Age; D006801:Humans; D015994:Incidence; D008137:Longitudinal Studies; D008423:Maternal Age; D008875:Middle Aged; D009035:Mothers; D009517:New South Wales / Q000453:epidemiology; D011247:Pregnancy; D011256:Pregnancy Outcome; D047928:Premature Birth / Q000453:epidemiology*; D016016:Proportional Hazards Models; D012307:Risk Factors; D012907:Smoking / Q000009:adverse effects*; D013995:Time