Some studies point to periodontitis as a possible cause of preterm delivery. Although this has not been entirely proven, there are grounds for considering it a risk factor for premature birth. 

Preterm labour 

Definition The World Health Organization (WHO) defines preterm or premature birth as labour that occurs before 37 weeks of pregnancy are completed. 

Importance Preterm birth is a major health problem because:
• It is the leading cause of perinatal morality (death of a baby within the first 28 days of life).
• It constitutes a major cause of health problems for the child, both on the short and the long term (retinopathy, alterations in psychomotor development, respiratory problems...).
• It is highly prevalent: in developed countries it is estimated to have a prevalence of between 5 and 10% of all births and in Spain 7.9%. 

Despite the significant preventive effort that has been made in recent years to decrease this important prevalence via examinations throughout pregnancy, preventive measures and pregnancy care, the desired effect has not been achieved, and not only has this problem not diminished, but rather, it has increased in the last two decades. 

Why does periodontal disease favour preterm delivery 

Risk factors for preterm delivery 

Preterm birth is a multifactorial problem where the mother's genetics, age and life conditions, etc. all come into play. However, in a very high percentage of cases, no particular causal factor can be identified. Maternal infections have been proven to be one of the main factors triggering preterm labour. In this sense, intrauterine infection is responsible for 25-40% of all premature births. 

Periodontitis as a risk factor 

In this context, periodontal diseases have been suggested as a possible cause of preterm labour. These diseases are caused and perpetuated by anaerobic, gram-negative bacteria present in subgingival biofilm, and are therefore chronic infections that induce a destructive inflammatory reaction in the periodontium, which could lead to tooth loss. The first study linking periodontal disease to preterm births and low birth weight infants was published in 1996 (Offenbacher et al. 1996). Since then, multiple studies have been conducted yielding contradictory results, as some of these studies observed this association while others were unable to confirm it. 

To better understand how periodontal pathogens can affect the evolution of a pregnancy, we must know its physiology: as the foetus grows, the increase in need for nutrients and the lack of space begin to be critical for the survival of mother and foetus, and cytokine levels in the amniotic fluid (tumour necrosis factor alpha [TNF-α], interleukin 1ß [IL-1ß]) and inflammatory mediators (prostaglandin E2 [PGE2]) increase until a level is reached which induces the rupture of membranes and uterine muscle contraction. Thus, delivery is controlled by inflammatory signals, and the mechanism that stimulates delivery can be modified by external stimuli, such as infection and inflammation (Madianos et al. 2013). 

Moreover, during pregnancy, and due to a higher hormone level, estrogen and progesterone, changes occur in the periodontium, with increased vascularisation and increased vascular permeability in the gingival tissues. Periodontal bacteria and/or their components (for example, endotoxins such as lipopolysaccharides [LPS]) may enter the systemic circulation (bacteraemia), facilitated by hormone-induced changes in tissues, thereby reaching the fetoplacental unit, where they may colonise and cause a foetal inflammatory response against these pathogens, and the consequent release of cytokines and inflammatory mediators, which can trigger the premature rupture of membranes and uterine contraction. 

Oral health recommendations for pregnant women 

Pregnant women should undergo a complete oral assessment, including a periodontal examination to diagnose their periodontal condition: 

• Practitioners must inform periodontally healthy women about physiologic alterations that will occur in their gums. They must also instruct them on oral hygiene techniques, with special attention to interdental hygiene, and recommend that they return for a reassessment of their periodontal health in later stages of the pregnancy. 

• Women with gingivitis should follow the same guidelines, and their gingivitis must be treated. They should have exams throughout their pregnancy to detect and treat possible recurrence. 

• For women with periodontitis, non-surgical periodontal treatment will be performed to reduce subgingival biofilm and inflammation. This should ideally be done between weeks 14 and 20 (second trimester of the pregnancy).