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4. Preterm Labor
4.1 Definition
Preterm labor is defined as regular rhythmic uterine contractions resulting in cervical changes that start before 37 weeks of a viable pregnancy. Changes in the cervix include effacement (the cervix thins out) and dilation (the cervix opens so that the fetus can enter the birth canal) [1].
4.2 Introduction
Preterm birth is a global concern. Every year, an estimated 15 million babies are born preterm, and this number is rising [2]. In the year 2010, almost 24% of the total world’s preterm babies were born in India [3].
Preterm birth is the leading cause of neonatal deaths and the second leading cause of death after pneumonia in children under 5 years [2].
The prognosis for individual preterm infants depends primarily on gestational age at birth. Mortality rates vary from about 2% for newborns born at or after 32 weeks to more than 90% for those born at 23 weeks. Many survivors face a lifetime of handicap, including learning disabilities and visual and hearing problems.
4.3 Classification of Preterm Births
Extremely preterm (<28 weeks).
Very preterm (28–<32 weeks).
Moderate to late preterm (32–<37 weeks).
The problems of preterm babies vary according to the degree of prematurity, with the maximum burden of difficulty being faced by the extremely preterm ones.
4.4 Etiology of Preterm Births
Preterm labor is a heterogeneous process. Multiple variables may simultaneously be responsible for the onset of preterm labor, hence the observed relatively low predictive value of any given variable. Most preterm births happen spontaneously, but some are iatrogenic, whether for medical or non-medical reasons. Genetics may also play a role in the etiology. Comprehending the causes and mechanisms associated with preterm births will help in the development of solutions to prevent preterm birth.
4.4.1 Risk Factors for Preterm Birth
Socioeconomic factors: Stressful pregnancy for various reasons such as low socioeconomic status, unwed mothers or lack of family support, low pre-pregnancy weight, smoking, alcohol, and substance abuse during pregnancy.
Infections: Intrauterine infection either as ascending genital tract infection, including bacterial vaginosis, or as a part of systemic infection leading to chorioamnionitis.
Anatomic abnormalities of the uterus and cervix, either congenital (unicornuate, bicornuate, arcuate, or septate uterus) or acquired (following cervical conization or LEEP or DES exposure in utero).
History of previous preterm births or recurrent abortions.
Multiple pregnancies.
Trauma.
Iatrogenic: Antepartum hemorrhage, diabetes mellitus, hypertensive disorders of pregnancy, renal diseases, and immunological disorders.
Idiopathic: Accounts for about 30% of preterm births.
The two strongest risk factors for idiopathic preterm labor are low socioeconomic status and previous preterm delivery. However, a history of prior preterm birth is not useful in the nulliparous patients who make up nearly one half of all patients experiencing preterm birth.
Around 25% of preterm deliveries are elective due to either maternal factors such as pre-eclampsia or fetal factors such as extreme growth restriction.
4.4.2 Infection
Intrauterine infection is a chronic process accounting for 25–40% of preterm births. Preterm labor is triggered by the activation of the innate immune system. However, the role of infections in causing preterm births decreases as the gestational age advances.
Decidual colonization by microorganisms can occur by several ways: ascending, hematogeneous, iatrogenic by procedures such as amniocentesis, or retrograde through the Fallopian tubes. From the decidua, infection may reach the space between the amnion and chorion, the amniotic fluid, and the fetus.
Asymptomatic colonization of the decidua occurs in up to 70% of women at term, only few of these experience preterm labor pains. Gardnerella vaginalis, Fusobacterium, Mycoplasma hominis, Ureaplasma urealyticum, Mycoplasma genitalium, and severe untreated Candida infection are associated with increased risk of preterm birth.
Endotoxins released by microorganisms and cytokines stimulate decidual responses including the release of prostaglandins which stimulate uterine contractions. The decidua can also release matrix-degrading enzymes that weaken fetal membranes leading to premature rupture. Bacterial vaginosis before or during pregnancy can aggravate the decidual inflammatory responses, and prophylactic antibiotic therapy is associated with decreased preterm birth incidence and complications.
Chorioamnionitis can lead to maternal and fetal sepsis and significant long-term fetal sequelae including cerebral palsy.
4.5 Diagnosis
Initial symptoms of preterm labor can be confused with the discomfort associated with gravidity. Hence, many women experiencing preterm labor present only in the late stages, while many healthy women may have multiple prenatal visits with similar complaints. A confirmatory diagnosis of preterm labor is made only in the presence of cervical changes, either on clinical examination or ultrasonography.
4.5.1 Symptoms and Signs of Preterm Labor
Increase in the amount of vaginal discharge.
Change in type of vaginal discharge (watery, mucus, or bloody).
Pelvic or lower abdominal pressure.
Constant low, dull backache.
Mild abdominal cramps, with or without diarrhea.
Regular or frequent uterine contractions, often painless.
Ruptured membranes (amniotic membrane breaks with a gush or a trickle of fluid comes out).
Vaginal spotting or bleeding.
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