bubble_chart Overview

Premature labor refers to childbirth occurring between the 28th and 37th weeks of pregnancy (196 to 258 days). Literature reports that premature labor accounts for 5% to 15% of all childbirths. Newborns delivered during this period, weighing between 1000 and 2499 grams with immature organs, are referred to as premature infants. The mortality rate for premature infants is 12.7% to 20.8% domestically, while internationally, the mortality rate increases with lower gestational age and birth weight. The main causes of death include perinatal asphyxia, intracranial hemorrhage, and congenital malformations. Even if premature infants survive, many experience neurodevelopmental and intellectual disabilities. Therefore, preventing premature labor is one of the key measures to reduce perinatal mortality and improve neonatal outcomes.

bubble_chart Etiology

Approximately 30% of premature labor has no obvious cause. Common triggers include:

1. Maternal factors

(1) Associated uterine abnormalities (such as bicornuate uterus, septate uterus), cervical incompetence, or uterine fibroids.

(2) Associated acute or chronic diseases, such as viral hepatitis, acute nephritis or pyelonephritis, acute appendicitis, viral pneumonia, high fever, rubella, and other acute diseases; chronic conditions like heart disease, diabetes, severe anemia, hyperthyroidism, hypertension, asymptomatic bacteriuria, etc.

(3) Complications like pregnancy-induced hypertension syndrome.

(4) Smoking, drug abuse, alcoholism, or grade III malnutrition.

(5) Others, such as long-distance travel, climate changes, living in high-altitude areas, family relocation, extreme emotional fluctuations, and other mental or physical burdens; direct abdominal impact, trauma, sexual intercourse, or surgical stimulation.

2. Fetal and placental factors

(1) Placenta previa and placental abruption.

(2) Polyhydramnios or oligohydramnios, multiple pregnancies.

(3) Fetal abnormalities, intrauterine fetal death, or abnormal fetal position.

(4) Premature rupture of membranes, chorioamnionitis.

bubble_chart Clinical Manifestations

Premature labor is similar to late abortion and also has its developmental process, which can be clinically divided into two stages:

1. Threatened premature labor: Uterine contractions occur, with at least one contraction every 10 minutes, each lasting 30 seconds, persisting for more than one hour.

2. Inevitable premature labor: In addition to regular uterine contractions, with progressively shorter intervals, longer durations, and increasing intensity, it is accompanied by cervical effacement ≥75% and cervical dilation ≥2cm; or there is progressive cervical effacement and dilation, along with bloody vaginal discharge or rupture of the fetal membranes, resembling the clinical presentation of full-term pregnancy.

bubble_chart Diagnosis

Uterine contractions and the progress of labor merely indicate that the pregnancy is nearing its end. To determine whether it falls under the category of premature labor, the key lies in confirming the gestational age and fetal size. Clinically, the following methods can be used to estimate gestational age and fetal size:

1. Clinical Estimation: - Obtain a detailed history of previous menstrual cycles, including the date of the last menstruation, the onset of early pregnancy symptoms, and the time when fetal movements began. - Assess whether the size of the uterus during early gynecological examinations corresponds to the duration of amenorrhea. - Estimate gestational age based on the current length of the uterus above the pubic symphysis and abdominal circumference.

2. Ultrasound Examination: - Fetal head diameter, head circumference, abdominal circumference, and femur length are closely related to gestational age and weight. - Ultrasound measurements can be used to estimate gestational age and fetal size. - Biparietal diameter (BPD) measurement is relatively accurate with minimal error. For example, if BPD ≥ 85mm, 96% of fetuses weigh ≥ 2500g. - Femur length measurement has a reliability of approximately 90%. For instance, if femur length ≥ 6.8cm, fetal weight is ≥ 2500g (Tables 15-1, 2, 3).

Table 15-1 Normal Pregnancy Fetal Biparietal Diameter Values (cm)

Gestational Week	Mean ± Standard Deviation	Gestational Week	Mean ± Standard Deviation
11	2.08 ± 0.577	26	6.31 ± 0.773
12	2.35 ± 0.525	27	6.67 ± 0.820
13	2.58 ± 0.515	28	7.09 ± 0.403
14	3.03 ± 0.757	29	7.23 ± 0.682
15	3.45 ± 0.580	30	7.39 ± 0.802
16	3.79 ± 0.358	31	7.93 ± 0.636
17	4.10 ± 0.820	32	7.94 ± 0.580
18	4.28 ± 0.406	33	8.13 ± 0.367
19	4.26 ± 0.630	34	8.30 ± 0.628
20	4.68 ± 0.711	35	8.47 ± 0.614
21	4.79±0.681	36	8.52±0.515
22	5.15±0.568	37	8.71±0.566
23	5.47±1.000	38	8.88±0.354
24	5.80±0.704	39	8.91±0.536
25	5.81±1.380	40	9.09±0.429

Table 15-2 Normal mean values of head circumference (HC) and abdominal circumference (AC) (cm)

Gestational week	HC	AC	Gestational week	HC	AC
12	7.0	5.6	27	25.2	22.9
13	8.9	6.9	28	26.2	24.0
14	9.8	8.1	29	27.1	25.0
15	11.1	9.3	30	28.0	26.0
16	12.4	10.5	31	28.9	27.0
17	13.7	11.7	32	29.7	28.0
18	15.0	12.9	33	30.4	29.0
19	16.3	14.1	34	31.2	30.0
20	17.5	15.2	35	31.8	30.9
21	18.7	16.4	36	32.5	31.8
22	19.9	17.5	37	33.0	32.7
23	21.0	18.6	38	33.6	33.6
24	22.1	19.7	39	34.1	34.5
25	23.2	20.8	40	34.5	35.4
26	24.2	21.9

Table 15-3 Normal Femur Length Values (cm)

Gestational Week	Femur Length	Gestational Week	Femur Length	Gestational Week	Femur Length
12	0.8	22	3.9	32	6.3
13	1.1	23	4.2	33	6.5
14	1.5	24	4.4	34	6.6
15	1.8	25	4.7	35	6.8
16	2. 1	26	4. 9	36	7. 0
17	2. 4	27	5. 2	37	7. 2
18	2. 7	28	5. 4	38	7. 3
19	3. 0	29	5. 6	39	7. 5
20	3. 3	30	5. 8	40	7. 6
21	3. 6	31	6. 1

bubble_chart Treatment Measures

1. Management of threatened premature labor

(1) Left lateral position to increase uteroplacental blood flow, reduce uterine activity, and relax uterine muscles, thereby decreasing spontaneous contractions.

(2) Intravenous infusion of 500–1000 ml of balanced solution to expand uteroplacental blood perfusion and reduce uterine activity, administered at a rate of 100 ml/h.

(3) While performing the above measures, conduct a rectal or vaginal examination to assess cervical effacement and dilation. After 1–2 hours of observation, if contractions become less frequent or disappear, no further examination is needed to avoid stimulating the vagina and cervix, which may trigger prostaglandin and oxytocin secretion.

With these measures, 40–70% of patients recover without additional treatment. If there is no improvement, repeat the rectal or vaginal examination to determine whether the condition has progressed to inevitable premature labor and provide corresponding management.

2. Management of inevitable premature labor

(1) Pharmacological inhibition of contractions

1) Indications: The following conditions must be met to use tocolytics to prolong pregnancy for several days (to allow time for corticosteroids to promote fetal lung maturation) or weeks (to enable continued fetal growth in utero, thereby reducing neonatal mortality and morbidity): ① Clear diagnosis of inevitable premature labor; ② Pregnancy ≥ 28 weeks; ③ No contraindications to continuing the pregnancy; ④ Fetus can continue healthy growth; ⑤ Cervical dilation ≤ 4 cm, labor still in the latent phase or about to enter the active phase.

2) Drug selection and mechanisms: Based on their mechanisms, tocolytics can be divided into two categories: First category: Block or inhibit the synthesis or release of contraction-inducing substances, such as ethanol and prostaglandin synthase inhibitors. Second category: Alter uterine muscle responsiveness to contraction-inducing substances, such as magnesium sulfate, β₂-adrenergic receptor agonists, and antihypertensives. If labor progression cannot be halted, discontinue use immediately. Commonly used drugs include:

Indomethacin: Prostaglandins stimulate uterine contractions and cervical softening. Indomethacin inhibits prostaglandin synthesis by blocking prostaglandin synthase. The usual dose is 25 mg orally every 6 hours or 50 mg as a rectal suppository every 12 hours until contractions cease. Maternal side effects are minimal. For pregnancies < 34 weeks, fetal sensitivity to adverse effects is low, especially with short-term use, and does not typically lead to premature closure of the ductus arteriosus, pulmonary hypertension, heart failure, or death.

Magnesium sulfate: Magnesium ions compete with calcium ions for entry into the sarcoplasmic reticulum and directly act on muscle cells, reducing membrane potential and inhibiting contraction. The inhibitory effect is dose-dependent. At serum magnesium levels of 2–4 mmol/L (4–8 mEq/L), spontaneous uterine contractions and oxytocin-induced contractions are completely suppressed. The initial dose is 4 g in 100–250 ml of 5% glucose solution, infused intravenously over 30–60 minutes. Subsequently, 5–10 g of magnesium sulfate in 500 ml of 5% glucose solution is infused at 1–2 g/h until contractions cease or labor progresses significantly without response. Monitor closely for magnesium toxicity, including maternal respiration, knee reflexes, and urine output. Adjust the infusion rate if adverse effects like vomiting or fever occur. If grade I contractions recur, repeat administration is possible. Contraindicated in cases of severe myocardial damage, conduction block, or renal impairment. Avoid concurrent use with other respiratory depressants.

β₂-Adrenergic receptor agonists: β₂-receptors are mainly found in the smooth muscles of the uterus, blood vessels, bronchi, and diaphragm. The drugs act directly on the receptors on the smooth muscle cell membrane. After binding to the corresponding receptors, they activate adenylate cyclase, increasing the content of cyclic adenosine monophosphate (cAMP) in the smooth muscle cells, inhibiting the release of calcium from the sarcoplasmic reticulum, and reducing the calcium content in the cytoplasm, thereby relaxing the uterine muscles and inhibiting contractions. Additionally, due to the stimulation of β₂-receptors, vascular smooth muscle relaxes, leading to vasodilation, increased blood flow to the uterus and placenta, and reduced uterine activity, further relaxing the uterus. However, these drugs can cause adverse reactions such as nausea, dizziness, headache, increased heart rate, arrhythmia, hypotension, as well as hyperglycemia, hypokalemia, hypocalcemia, and hypomagnesemia. Currently, drugs used to treat premature labor include salbutamol sulfate, terbutaline sulfate, isoxsuprine, and ritodrine. Salbutamol sulfate has fewer cardiovascular adverse effects and is highly effective in inhibiting uterine contractions. The oral dose is 4.8 mg, and if no adverse reactions occur, another 2.4 mg is given after half an hour, followed by 2.4 mg after 8 hours, which can be repeated as needed. The dose of isoxsuprine is 0.5–1 mg/min intravenously until contractions stop, followed by the minimum effective dose maintained for 8–12 hours, then switched to oral medication, 5–20 mg every 3–6 hours. If intravenous infusion exceeds 1 hour and contractions persist, treatment should be stopped as it indicates failure. Terbutaline sulfate has a longer duration of uterine contraction inhibition and fewer cardiovascular adverse effects. The usual dose is 10 μg/min intravenously, gradually increased by 5 μg/min each time. After 1 hour, the dose is reduced by 5 μg/min every 30 minutes to the minimum effective dose, maintained for 8 hours. Subsequently, switch to subcutaneous injection of 250 μg every 6 hours for 3 days, then switch to oral administration of 5 mg every 8 hours until 36 weeks of gestation. The usual dose of ritodrine is 50 μg/min intravenously, increased by 50 μg every 10 minutes until contractions cease for 1 hour. If the heart rate exceeds 120 beats per minute, the dose is gradually reduced until the heart rate normalizes. Generally, intravenous administration should not exceed 12 hours. Half an hour before stopping intravenous administration, oral administration of 10 mg every 2 hours begins, continued for 24 hours, then gradually reduced to 20–60 mg daily, divided into 2–3 oral doses.

Calcium antagonists: Their primary action is to block calcium ions from entering the cell membrane, inhibit the release of calcium from the intracellular myofibril membrane, and increase calcium expulsion in smooth muscle, thereby reducing cytoplasmic calcium levels and relaxing the uterine muscle. Among these drugs, the most potent is nifedipine (heart pain medication). The dose is 10mg, taken orally three times daily; sublingual administration acts faster and can reduce the amplitude and muscle tension of uterine contractions. However, it may cause peripheral vasodilation, slowed atrioventricular conduction, subsequent reflex tachycardia, headache, skin flushing, and reduced uteroplacental blood flow.

(2) Drug-induced fetal lung maturation: If premature labor is deemed unavoidable, glucocorticoids should be administered intramuscularly, intravenously, or via intra-amniotic injection alongside tocolytics to promote fetal lung maturation and prevent respiratory distress syndrome in premature infants, thereby improving their survival rate. Commonly used dexamethasone is given at 5mg intramuscularly three times daily for 2–3 days, or betamethasone at 12–24mg intramuscularly once daily for 2 days. The effect generally occurs within 24–72 hours.

3. Management of childbirth: The focus is on avoiding traumatic delivery, neonatal asphyxia, and ensuring thorough preparation for resuscitation and warmth after birth.

(1) Oxygen administration.

(2) During the first stage of labor, position the laboring woman on her left side to increase placental perfusion.

(3) Avoid the use of sedatives and analgesics.

(4) Administer vitamin K ₁ 10mg intramuscularly to reduce the incidence of neonatal intracranial hemorrhage.

(5) Upon entering the second stage of labor, perform an episiotomy under pudendal nerve block anesthesia at the appropriate time to reduce resistance from the pelvic floor against the fetal head. If necessary, prophylactic forceps-assisted delivery may be performed, but the procedure must be gentle to avoid injuring the fetal head.

4. Management of premature infants

(1) Immediate postnatal care

1) Positioning: To prevent backflow of neonatal blood to the placenta, position the infant below the level of the placenta after delivery. To facilitate the drainage of mucus, blood, and amniotic fluid from the oropharynx, place the newborn face-down or in a supine position with the head tilted to one side. Use saline gauze to gently squeeze the nostrils and clean the mouth.

2) Airway clearance: It is crucial to remove mucus, blood, and amniotic fluid from the respiratory tract before the first breath. Extend the newborn’s head and use electric suction or a mouth-held catheter to clear fluids from the oropharynx, then gently tap the soles to stimulate crying. The adaptability of premature infants to extrauterine life varies with gestational age and birth weight. If placental function was adequate before birth, most can adapt to the new environment and begin spontaneous breathing within 1–2 minutes after delivery. However, if the birth weight is too low (<2000g), the respiratory center in the medulla may respond poorly to physical and chemical stimuli. Additionally, premature infants are prone to brain injury during delivery, and factors such as immaturity, hypoxia, and intracranial hemorrhage may contribute to a sluggish respiratory response. Weak chest muscles may also fail to sustain adequate respiratory movements, leading to alveolar collapse and respiratory distress. In cases of pale asphyxia, rapid endotracheal intubation, suction of tracheal fluids, oxygen administration, and positive-pressure ventilation are required. The longer the delay in establishing pulmonary respiration after birth, the higher the likelihood of permanent central nervous system damage.

3) Umbilical cord clamping: Clamp the cord immediately after airway clearance and resuscitation to reduce the risk of hyperbilirubinemia and liver burden.

4) Thermal care: After cord clamping, quickly dry the infant’s body but avoid removing the vernix caseosa, which provides thermal protection. Wrap the infant in warm, dry cloth to minimize heat loss.

(2) Postnatal management

1) Keep warm: Maintain room temperature at 24～26℃ with relative humidity of 55～65%. The lower the body weight, the closer the ambient temperature should be to the premature infant's body temperature. Premature infants weighing <2000g should be placed in an incubator. For those weighing 1501～2000g, the incubator temperature should be 30～32℃; for those weighing 1001～1500g, the incubator temperature should be 32～34℃.

2) Daily Care: In addition to measuring weight once daily at a fixed time (before feeding), all nursing tasks such as feeding, temperature measurement, changing clothes and diapers should be completed within the incubator. Avoid unnecessary examinations and movements. Initially, measure axillary temperature every 2 hours, and after the temperature stabilizes, measure it every 4 to 6 hours. The temperature should be maintained at a skin temperature of 36–37°C and a rectal temperature of 36.5–37.5°C.

3) Oxygen Supply: Oxygen should only be administered when cyanosis and respiratory distress occur, and prolonged use should be avoided. The oxygen concentration should be maintained at 30–40%. Excessive concentration or prolonged oxygen administration may lead to retrolental fibroplasia, causing visual impairment.

4) Prevention of Hypoglycemia: Statistics show that approximately half of premature infants develop hypoglycemia within the first day after birth. If blood glucose levels are below 1.1 mmol/L (20 mg/dL) twice after birth, immediate treatment is required. Intravenous glucose at 1 g/kg can be administered, followed by continuous infusion at 10 mg/kg per minute. Once blood glucose stabilizes, continue the infusion for 24 hours, then gradually reduce the dosage based on feeding conditions.

5) Vitamin and Iron Supplementation: Premature infants have low reserves of various vitamins, rapid growth, and high demands, making them prone to deficiencies. Therefore, after birth, vitamin K₁ (1–3 mg) and vitamin C (50–100 mg) should be administered intramuscularly or intravenously for 2–3 days. Starting from the third day, oral multivitamin B (half a tablet) and vitamin C (50 mg) should be given twice daily. From the 10th day, concentrated cod liver oil drops can be administered, gradually increasing from 1 drop daily to 3–4 drops daily, or a single intramuscular injection of vitamin D₃ (150,000–300,000 IU). At one month of age, iron supplementation (10% ferric ammonium citrate at 2 mL/kg daily) should be provided. For infants with a birth weight <1,500 g, vitamin E (30 mg daily) should be given from the 10th day for 2–3 months.

6) Feeding: Breastfeeding should begin 6 hours after birth, with 1–2 test feeds of sugar water before milk. For infants with very low weight or poor general condition, feeding may be appropriately delayed, and intravenous fluids should be provided. For those with weak sucking ability, feeding via gastric or intestinal tube may be necessary. Premature infants have significant individual differences in caloric and fluid requirements. For most infants in the first week, calories can be calculated at 502.32 kJ/kg (120 kcal/kg) daily, and fluids at 60–80 mL/kg daily.

7) Infection Prevention: Strengthen daily cleaning and disinfection in the premature infant ward and strictly enforce isolation protocols. If a premature infant shows signs of infection, prompt treatment should be administered.

bubble_chart Cure Criteria

Able to suckle on their own, maintaining stable body temperature in a normal room environment, with steady weight gain of 10-30g/day and already exceeding 2,000g. Medications and oxygen therapy have been discontinued for some time. For those who previously received oxygen therapy, an eye examination should be performed to rule out retinopathy of prematurity, and a routine hemoglobin test should be conducted.

bubble_chart Differentiation

During the advanced stage of pregnancy, the sensitivity and contractility of the uterus gradually increase, often leading to contractions after fatigue or prolonged walking. However, these contractions subside quickly with brief rest and differ from the clinical manifestations of threatened premature labor. As for inevitable premature labor, it must be distinguished from false labor contractions. False labor contractions are characterized by long and irregular intervals, short and inconsistent durations, and no increase in contraction intensity. They often occur at night and disappear by morning. These contractions only cause mild distending pain in the lower abdomen, without shortening of the cervical length or significant dilation of the cervical os, and can be suppressed by sedatives.