This study provides evidence that C. trachomatis infection during pregnancy is associated with preterm delivery, but not with low birthweight or being small for gestational age. Young age, Antillean ethnicity and single marital status were independent risk factors for C. trachomatis infection in pregnant women.
The strength of this study is its population-based, non-interventional and prospective design with a large number of well-described participants, adjustments for many potential confounders, use of a highly sensitive microbiological test method and near perfect follow-up until delivery (97%).
A potential weakness is that the Generation R cohort is slightly skewed towards a relatively affluent, healthy study population [30
]. However, we do not expect this to affect our results since the study was designed to assess the effects of Chlamydia on pregnancy outcomes in a non-hospital based, low-risk population. Another weakness could be that not all women initially enrolled in the overall study could be tested for C. trachomatis
. We had a non-response of 9.5% (491 women) for the Chlamydia study. This may in part be true non-response, but is more likely due to a change in the Generation R routine with a later start of inclusion of urine collections for the Chlamydia study. Another 12.0% (621 women) had to be excluded because their urines could not be matched to the respective questionnaires in the database. This was due to logistical problems in the pilot phase of the Chlamydia study. However, all risk factors were similarly distributed among tested and untested women and no differences were found in median gestational age and mean birthweight (39.8 weeks versus 39.8 weeks; P
= 0.84, 3,408 g versus 3,407 g; P
= 0.92). Likewise, no differences were observed between chlamydia-positive and negative women regarding the measurement and calculation of gestational age at the prenatal visits. Furthermore, our numbers were too small to assess an association of Chlamydia with miscarriage or perinatal death. Of all enrolled women with C. trachomatis
results, 62 (1.5%) had a miscarriage or perinatal death. Data on gestational age and birthweight were not available for these deliveries, but likely is that these women delivered relatively more often prematurely. None of these women tested positive for C. trachomatis
. Theoretically, our effect estimates for associations of chlamydial infection with gestational age and birthweight could be biased and exaggerated when these associations would differ between all fetuses and fetal `survivors`. This would be the case if C. trachomatis
infection would have a `protective` effect on early fetal death. However, this is most unlikely [8
Previous studies of associations between maternal chlamydial infection and subsequent spontaneous preterm delivery produced mixed results. Most of these studies were small compared to our study (less than 1,000 women included) and were published 10 to over 20 years ago. Early studies were often based on serology, which does not reliably distinguish current from past infection [9
]. One case–control study found an association between the presence of IgM anti-chlamydial antibody and preterm delivery [11
]. Studies using cervical culture for C. trachomatis
at that time also yielded conflicting results [9
]. More recently, sensitive DNA amplification techniques have been used to screen pregnant women for C. trachomatis
, but again with contradicting results regarding preterm delivery [21
]. Using this methodology one case–control study, nested in a USA study among 2,929 pregnant women, reported a two- to three-fold increased risk of preterm delivery before 35 weeks gestation [21
]. Later, the same group reported from an observational study among 2,470 women with bacterial vaginosis or Trichomonas vaginalis
infection enrolled in a multicentre randomized antibiotic treatment trial that Chlamydia in midterm pregnancy was not associated with increased risk of preterm delivery [23
]. However, most of their chlamydia-positive women were treated between 16–29 weeks gestation. Three more studies in which chlamydia-positive pregnant women were treated also found no association with prematurity or a much lower relative risk for prematurity (1.50) than we found in our study [24
]. In a prospective South-African study among low-risk pregnant women, chlamydia-positive women (cases, n
= 40) had a relative risk of 2.20 for preterm delivery before 37 weeks gestation (controls, n
= 303) [22
]. In this study women were also tested for Syphilis, Gonorrhoea and bacterial vaginosis, which were found not to be associated with preterm delivery. Likewise, one of the former studies also found no association between preterm delivery and syphilis, bacterial vaginosis, Trichomonas vaginalis
and Neisseria gonorrhoeae
]. The true effect of C. trachomatis
on pregnancy outcomes therefore cannot be fully ascertained from studies in which pregnant women are screened for Chlamydia and treated in the first phases of their pregnancy. In contrast, our study is prospective, non-interventional and population-based (n
= 4,055), which makes that our findings better estimate the effect of Chlamydia on pregnancy and are more representative for the population as a whole.
Preterm birth represents a major problem for obstetrics and neonatology due to its increasing frequency and accompanying socio-economic impact. We found chlamydial infection to be associated with preterm delivery before 32 and 35 weeks gestation, which was much stronger for the former indicating that Chlamydia contributes relatively more to early than late prematurity. For such an important issue, a four-fold increased risk of preterm delivery before 32 weeks gestation implies that a considerable proportion (14.9% in our cohort) of preterm deliveries before 32 weeks gestation is attributable to chlamydial infection in pregnancy. Despite a considerable confidence interval around the estimate, this would classify C. trachomatis
among the important infective risk factors of early prematurity. The attributable fraction remained significant (7.4%) for preterm delivery before 35 weeks. It should be noted, however, that the number and proportion of premature deliveries attributable to Chlamydia highly depends upon the C. trachomatis
prevalence in a given population, and that some residual confounding as a result of co-infection by other genitourinary pathogens cannot be excluded. Another limitation could be that we did not correct for other potential risk factors as previous preterm delivery, since we did not have such data, and previous termination of pregnancy in these women since this potential confounder was only recently established [36
]. Importantly, we had no information concerning the use of (macrolide) antibiotics during pregnancy, but suggest that this would mitigate, rather than exaggerate, the effect of Chlamydia on pregnancy outcome. The latter information would have been interesting since early treatment may prevent preterm birth (34, 35). Finally, pregnant women are not routinely tested and treated for Ureaplasma urealyticum
or Mycoplasma genitalium
in the Netherlands and neither are women with premature rupture of the membranes routinely treated with antibiotics.
Extrapolation of our findings to the Netherlands, where approximately 3,000 neonates are born before 32 weeks gestation annually [37
], showed that C. trachomatis
infection in pregnancy contributes approximately 450 cases to this burden; for deliveries before 35 weeks gestation these numbers are 7,100 and 525, respectively. Our finding of an association between C. trachomatis
infection and premature delivery may be useful in a cost-benefit analysis of Chlamydia screening during pregnancy.
Interestingly, C. trachomatis infection was more prevalent among women who had twins than those who had singletons, a finding not reported before. Since Chlamydia is associated with infertility and assisted conception for infertility is associated with twin pregnancies, an explanation for our findings may be that chlamydia-positive women were not treated effectively during their infertility work-up or that they were reinfected. This observation warrants further study into medical histories of women with twin deliveries versus women with singletons.
The major risk factors we found are amongst the many previously described including young age, urban residence, low socio-economic class, specific ethnic groups, single marital status and recent changes in sexual partnerships or sexual promiscuity [9
]. Adolescents are at highest risk of infection, which was also observed in this study [27
]. We could not evaluate urban residence since all participating women resided in Rotterdam. The risk factors we found are similar to those reported in another Dutch community-based study [28
]. Importantly, C. trachomatis
prevalences vary widely [26
], which will directly affect the incidence of complications attributable to this infection, including preterm delivery.
One may question the design of our study since women screened and found positive for C. trachomatis
were not treated. The design of our study would face ethical barriers in countries that have national guidelines or directives advocating Chlamydia screening and treatment in the routine of antenatal care, and would be impossible to perform [41
]. However, in most countries of the European Union, including the Netherlands, screening and treatment for Chlamydia during pregnancy remains controversial and is currently not recommended in routine antenatal care [43
]. Our study provides novel data that may well have an impact on the future antenatal screening strategy in the Netherlands and elsewhere. In addition, Generation R was designed as a non-interventional follow-up study [29
]. All following substudies, including ours, had to fit into this overall design. Furthermore, Generation R provided the data anonymously to protect the privacy of participants. We, therefore, had no access to names or addresses of chlamydia-positive women.
In conclusion, C. trachomatis urogenital infection in pregnant women increases the risk of preterm delivery, especially early prematurity, such that a significant proportion of preterm deliveries can be attributed to this infection. In order to improve birth outcomes, health systems should consider additional focus on C. trachomatis infection, especially in young women and others at increased risk.