Classification of preterm birth (PB)
In global clinical practice, it is customary to divide cases of late miscarriage depending on gestational age (according to WHO criteria) and fetal body weight.
By gestational age:
- Extremely early: 22 — 27 weeks 6 days (accounting for 5–7% of all PTB).
- Very early: 28 — 31 weeks 6 days.
- Early: 32 — 33 weeks 6 days.
- Late: 34 — 36 weeks 6 days.
By newborn body weight:
- Extremely low: from 500 to 1000 g.
- Very low: from 1000 to 1500 g.
- Low: from 1500 to 2000 g.
By the mechanism of development, induced PTB (30–40%), which are performed for medical indications, and spontaneous PTB (60–70%) are distinguished. Spontaneous PTB may begin either with the development of regular uterine activity or with premature rupture of membranes (PROM).
Etiology and pathogenesis
The diagnosis of miscarriage is impossible without understanding the causes triggering the process. Genital tract infections play a key role. Pathogenic microflora causes the release of cytokines and activates the "arachidonic cascade". This leads to the synthesis of prostaglandins, which stimulate myometrial contractions and provoke rupture of the fetal membranes.
Additional triggers:
- Progesterone deficiency (the main pregnancy-maintaining hormone).
- Increased number and sensitivity of oxytocin receptors in the uterus.
- Hereditary or acquired hemostatic disorders causing microthrombosis in the placenta.
- Immunological conflict between the maternal organism and the fetus.
As a result of these processes, inflammatory changes in the fetal membranes develop (including decidua — the decidual membrane), uterine tone increases, and isthmocervical insufficiency (ICI) is formed.
Risk groups: The physician must be vigilant if the patient has a burdened history (previous miscarriages or PTB), extragenital pathologies (hypertension, heart disease, thrombophilia), uterine malformations, or uterine overdistension (multiple pregnancy, polyhydramnios). Pregnant women after assisted reproductive technologies (ART) are also at risk.
Clinical presentation and diagnostics
Diagnostic workup is based on a comprehensive assessment of complaints, risk factors, as well as laboratory and instrumental data. During external obstetric examination, the physician determines increased uterine excitability and tone.
Clinically, spontaneous pregnancy termination is divided into two stages, the boundary between which is determined by the severity of contractions and the dynamics of cervical dilation:
- Threatened PTB:
- Complaints: dragging pain in the lower abdomen and lower back.
- Objectively: palpation or tocography records increased myometrial tone.
- Vaginal examination: the cervix is shortened, the external os may be slightly open.
- Incipient PTB:
- Complaints: onset of regular contractions.
- Vaginal examination: the cervix is effaced or significantly shortened, the os is dilated by 2–3 cm.
- Often accompanied by premature rupture of membranes.
Obstetric management: pessary and medical support
When cervical incompetence (ICI) is detected, one of the effective methods for prolonging pregnancy is the placement of an obstetric support pessary.
Its main advantages are safety, ease of placement (does not require anesthesia), cost-effectiveness, and the possibility of outpatient use. Unlike surgical cerclage, the pessary can be used at gestations over 22 weeks, when surgery carries high risks of complications.
Drug therapy after ICI correction includes:
- Gestagens (dydrogesterone or vaginal progesterone).
- Tocolytics ($\beta$-adrenergic agonists, calcium channel blockers) to reduce tone.
- Antibiotics (prescribed strictly when there is a high risk of infection and taking into account the sensitivity of the vaginal microflora).
Scheduled removal of the pessary or sutures is performed at 37 weeks. Emergency removal is indicated at any gestation in case of bleeding, rupture of membranes, onset of active contractions, signs of chorioamnionitis, or the need for urgent delivery.
Briefly: answers to topical questions
Which maternal extragenital pathologies are risk factors for preterm birth?
Extragenital diseases classified as risk factors for preterm birth include:
- arterial hypertension;
- hyperthyroidism;
- heart disease;
- anemia;
- thrombophilia.
What instrumental and laboratory diagnostic methods are used to confirm the threat of preterm birth?
Diagnosis of threatened preterm birth is based on a comprehensive assessment of risk factors, patient complaints, and laboratory and instrumental data.
The following are used:
- external obstetric examination — reveals increased uterine tone and excitability;
- tocography — allows registering increased uterine tone;
- transvaginal sonography (cervicometry) — used to assess the condition and length of the cervix; in high-risk patients, a cervical length of less than 25 mm detected at 16–24⁶ weeks is one of the criteria for a high risk of preterm birth.
Which specific drugs from the groups of beta-adrenergic agonists and calcium channel blockers are used as tocolytics?
The following specific drugs from the specified pharmacological groups are used as tocolytic therapy:
- Hexoprenaline — belongs to the group of $\beta_2$-adrenergic agonists. The drug is administered intravenously (with the woman lying on her left side), while cardiac monitoring of the patient's and fetus's condition is desirable.
- Nifedipine is a calcium channel blocker (calcium antagonist). It is administered orally according to a special regimen. The drug has an off-label status (not registered as a tocolytic), so its prescription requires a medical board conclusion and the patient's written informed consent.
What are the contraindications to tocolytic therapy?
Contraindications to tocolytic therapy include:
- intrauterine fetal death;
- congenital malformations incompatible with life;
- severe preeclampsia;
- hemorrhage with hemodynamic instability;
- chorioamnionitis;
- placental abruption with normally positioned placenta;
- uterine scar dehiscence;
- cervical dilation greater than 4 cm;
- preterm premature rupture of membranes at gestational age greater than 32 weeks;
- medical contraindications to a specific drug.
Tocolysis is not performed at a gestational age of 34 weeks or more, as well as when there are indications for emergency delivery.
What ultrasound (cervicometry) criteria reliably indicate cervical incompetence?
In high-risk patients, the criterion for cervical shortening according to transvaginal cervicometry is a cervical length of less than 25 mm detected between 16⁰ and 24⁶ weeks of gestation; a high risk of preterm birth is determined by the combination of this sign with a burdened medical history.
Measurement of funneling of the internal os, fundal pressure, and coughing have no diagnostic significance when the cervical length is preserved.
To remember the infectious pathogenesis of miscarriage, use the ICPS chain: Infection → Cytokines → Prostaglandins → Uterine contractions and rupture of membranes.
Class and exam questions
How is gestational age correctly calculated for making a diagnosis?
Gestational age is always calculated from the first day of the last normal menstrual period.
What is the main difference between threatened and onset preterm labor?
The main criterion is the dynamics of cervical changes. In threatened labor, the cervix merely shortens, while in active labor, regular contractions appear and dilation reaches 2–3 cm.
When is it necessary to urgently remove an obstetric pessary?
The pessary is immediately removed upon the appearance of spotting, rupture of amniotic fluid, development of regular labor activity, or the onset of infection (chorioamnionitis).
What else to study on this topic
- Complications in the premature newborn (intracranial hemorrhage, necrotizing enterocolitis).
- Risk groups for disability in children with extremely low birth weight (cerebral palsy, retinopathy, bronchopulmonary dysplasia).
- Indications for induced preterm labor from the maternal and fetal sides.
- Labor anomalies in miscarriage (monotonous contractions, rapid cervical dilation).
- Rules for collecting material for microbiological examination before prescribing antibiotics.
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