Out of all the anthropoid species the human neonate is born much more immature, perhaps its nature's mechanism to avoid the large head of the human fetus from becoming impacted in small pelvis of the mother who has adopted to a bipedal gate.
DEFINITION
- As per World Health Organization and Federation of International Gynecologist and Obstetricians, spontaneous preterm labor (PTL) is “labor resulting in birth before 37 completed weeks (259 days) of gestational age, based on the first day of last menstrual period.”
- This definition has no functional basis and must not be confused with prematurity, which is the lack of development of various organ systems especially lungs leading to respiratory distress syndrome at the time of birth
- The concept of completed weeks should not be misunderstood while truncating it to a round of form of weeks. For example, a woman with 36 weeks plus six days would be considered as 36 completed weeks and not 37. Due to this error up to 10% of preterm babies are misclassified.
Classification of Preterm Birth
Classification remains elusive. No classification copes really well with fetal death, which occurs before preterm labor without any recognizable cause.
The most practical classification is:
- Very early preterm birth: 20–27 weeks
- Early preterm birth: 28–31 weeks
- Late preterm birth: 32–37 weeks
The main concern and burden comes from those who are born before 33 weeks (90%).
EPIDEMIOLOGY
- The burden of PTL is much more in developing countries like India
- There are no accurate recent world wide data regarding the prevalence of PTL, but estimate ranges from 5% in developed countries to 25% in third world countries. The prevalence has been static in developed countries for decades and is in the range of 5–10%
- Twenty eight of total early neonatal deaths (deaths within seven days) are due to preterm birth
- The children who survive are major financial burdens due to the morbidity associated with physical and psychological issues
- Is preterm birth declining or increasing: Though the incidence of preterm birth has shown to be increasing especially in the countries where there has been no major change in birth registration system, for example, United States and Canada, but events leading to this rise are not clear and well understood. Etiology is thought to be multifactorial, whether these factors play independently or work in unison is still not clear. Two important factors, however, are increased in ART pregnancies and birth of higher order.
Causes of Preterm Labor
Epidemiological model of preterm labor: Hogue and colleagues have proposed an epidemiological model based on host, environment, and agent triangle. The host here is the individual woman, susceptible to stress or induced pathology, environment includes social and cultural conditions that are ongoing stressors. The agent is the immediate environmental or physical stressor requiring her response. This model tries to clarify whether the individual is overwhelmed by stressor depends not only on the strength but on host susceptibility for stress as well as the background level of acute, environmental and contextual stressors and that's why different women react differently to these situations:
- Racial variations: There are ethnic variations in preterm birth rates and it is higher in black population. This could be associated with an accelerated rate of maturity in black fetus and neonate. Surprisingly, the gestation specific mortality rate is lower in these neonates who are born before 38 completed weeks (Fig. 1)
- Socioeconomic status: The rate of preterm birth is 4% in higher classes versus 6% in lower socioeconomic class. Marital status and smoking habits also influence it
- There are conflicting studies regarding the association of BMI and preterm birth but majority quote an association, preterm birth is around 4% amongst the women with BMI 25–26, which increase up to 5% in overweight women and goes up again up to 7% if the BMI is <17
- There is a linear relationship with maternal height
- Maternal stress is a chronic and an independent factor.
- Causes could be divided into two groups—spontaneous and iatrogenic
- About 45–50% preterm deliveries are idiopathic, followed by PROM in 30%. Around 15–20% are medically indicated planned preterm deliveries (Fig. 2)
- Iatrogenic preterm delivery is the main cause in developed countries. These births take place between 28–35 weeks mostly due to medical reasons like hypertension and pre-eclampsia
- The commonest etiological factor worldwide is infection. Chronic UTI is the commonest infection associated with preterm labor (predisposed mainly due to immnunosuppression because of malaria and HIV)
- Other responsible factors are, multiple pregnancies, which increase the probability of preterm birth up to 10-fold especially in very early and early preterm labor group, intrauterine growth retardation (IUGR), maternal stress and intense physical labor (Box 1)
- The main challenge is finding ways to distinguish and quantify very early, early and late preterm births, which will decide the terms of survival, short and long term morbidity and health resources investments.
- Categorization of preterm birth (spontaneous vs iatrogenic) is important as this will show the true global trend of PTL and at the same time, it will help in making policies and interventions to reduce the risk of preterm labor and deliveries
Public Health Perspective of Preterm Labor
Great improvements have been done in last decade for the survival of preterm infants who do not have any birth defects and this has directly influenced our concept of viability, and on decision-making for a very preterm infant. However, still short- and long-term morbidity, and cost of care, in all ‘senses’ to families and society remains a major problem. However, the optimistic thing is that most preterm deliveries are occurring in 32–36 week group and their mortality and morbidity is really very low.
SUGGESTED READINGS
- Beck S, Daniel Wojdyla, Lale Say, Ana Piler Betran, Mario Marialdi, Jennifer Harrisrequejo, Craig Rubens, Ram Kumar Menon, Paul Fu Vonlook, The worldwide incidence of preterm birth: A systemic review of maternal mortality and morbidity. Bull World Health Organ; 88(1) Genebra Jan 2010.
- Hogue CJ, et al. Stress and preterm delivery: a conceptual framework. Paediatr Perinat Epidemiol; 2001;15 suppl 2:30–40.
- Judith Lumley. Defining the problem: epidemiology of preterm birth. BJOG. 2003.
- Phillip Steer. The Epidemiology of preterm labour. BJOG. 2005.