Preterm Labor

◈ Preterm Birth
1. Definition
(1) Prematurity : Gestational age < 37wks
(2) Low birth weight (LBW) : <2500g
(3) Very Low birth weight (VLBW) : <1500g
(4) Extremely Low birth weight (ELBW) : <1000g
(5) Small for gestational age : birthweight below the 10th percentile
(6) Appropriate for gestational age : between the 10th and 90th percentiles
(7) Large for gestational age : birthwight above the 10th percentile

2. Causes of preterm birth

1) Medical and obstetrical complications
* 28% - preeclampsia, fetal distress, fetal growth restriction, abruptio placentae, fetal death
* 72% - spontaneous preterm labor with or without ruptured membranes
+ 추가로 placenta previa, multiple gestation 또한 흔한 원인이다.
2) Lifestyle factors
: cigarette smoking, poor nutrition, poor weight gain during pregnancy, use of drugs such as cocaine or alcohol
: other maternal factors - young maternal age, poverty, short stature, occupational factors, psychological stress
3) Genetic factors
4) Chorioamniotic infection
: 20% of women - no evidence of overt clinical infection and with intact fetal membranes
: Pathophysiology (figure 28-8)
: amniocentesis - pregnancy outcome을 좋게 한다는 보고는 없지만 amniotic fluid WBC count, low glucose, high interleukin-6, Gram staining, Culture로 진단할 수는 있다.
: microorganism에 의해 membrane rupture, perterm labor로 진행할 수 있다.
5) Bacterial vaginosis
: the normal, hydrogen peroxide-producing lactobacillu-predominant vaginal flora is replaced with anaerobic bacteria(Gardnerella vaginalis, Mobiluncus species, Mycoplasma hominis)
: Mechanism은 amniotic fliud infection과 같다.
: Diagnostic feature
(1) Vagnial pH > 4.5
(2) An amine odor when vaginal secretions are mixed with potassium hydroxide
(3) Vaginal epithelial cells heavily coated with bacilli - 'clue cells'
(4) A homogeneous vaginal discharge
(5) Gram staining of vaginal secretion

3. Risk factors for preterm birth
1) Prior preterm birth

2) Cervical dilatation
: Asymptomatic cervical dilatation after midpregnancy
: Preterm birth의 27%정도가 26-30주에 2-3cm의 dilatation이 있음
3) Cervical length
: mean cervical length at 24 weeks = about 35mm
- cervical length가 짧을수록 preterm birth rate가 증가
그림 27-9
4) Sign & Symptoms
: painful or painless uterine contractions, pelvic pressure, menstrual-like cramp, watery or bloody vaginal discharge, and pain in the low back
=> impending preterm birth와 관련된 증상들로서 preterm labor 24시간 전에만 나타나기 때문에 late warning sign으로 볼 수 있다.
4) Fetal fibronectin
: detection of fetal fibronectin in cervicovaginal secretions prior to membrane rupture
( >50 ng/mL)
5) Ambulatory uterine contraction testing
: ↑ uterine activity beginning at about 30 weeks
: but home uterine activity monitoring is ineffective in the prevention of preterm birth

4. Management of preterm labor
=> correct identification & whether there is accompanying membrane rupture
1) Diagnosis
: True vs False labor
- True labor는 cervical effacement & dilatation을 동반함
- 그렇지만 term전에 분만하는 산모 중에 Braxton Hick contraction(irregular, nonrhythmical and either painful or painless)과 비슷한 양상을 보이는 경우가 드물지 않게 있어 false labor로 잘못 진단될 수도 있다.
: Criteria of preterm labor (American College of Obstetricians and Gynecologists, 1997)
(1) Contractions occurring at a frequency of four in 20 minutes or eight in 60 minutes plus progressive change in the cervix
(2) Cervical dilatation > 1cm
(3) Cervical effacement ≥80%

2) Management
=> intentional delivery를 해야 하는 maternal or fetal indication이 없는 한, management는 preterm birth를 억제하거나 태아가 extrauterine environment에 적절하게 대처할 수 있는 능력을 키워주는 것을 목표로 한다.
* Preterm premature rupture of the membrane(PPROM)
- Two primary forms of approach
(1) Nonintervention or expectant management, in which spontaneous labor is simply awaited
(2) Intervention that may include glucocorticoids, given with or without tocolytic agents to arrest preterm labor in order that the corticosteroids have sufficient time to induce fetal maturation
- Natural history of PPROM
: 입원당시 75%는 이미 labor 단계이고, 5%는 다른 complication으로 delivery, 10%는 48hr내에 spontaneous delivery, 7%만이 rupture된 후 48hr이상 지연될 수 있다.
: 대부분은 1주일 내에 labor에 들어간다. => ∴ Hospitalization 필요

* Preterm labor with intact fetal membranes
: PPROM과 거의 동일하고 가능하면 34주 이전에 delivery하지 않도록 하는 것이 중요
* Corticosteroid therpy
- Betamethasone(12mg IM in two doses 24 hr apart)
: 투여 시작 후 적어도 24시간동안 delivery를 억제하면 34주 미만에 태어난 infant에게서 RDS의 risk를 줄일 수 있다. 7일까지 투여하는 것이 완료하는 것.
- Adverse effect
: short-term maternal effects
- pulmonary edema, infection, difficult glucose control in diabetic women
* Antenatal phenobarbital and vitamin K therapy
- neonatal intracranial hemorrahge를 줄이기 위해 사용되지만, 효과는 별로 없는 것으로 보임
(Thorp et al. 1995)

* Tocolytic therapy
(1) Bed rest - no conclusive
(2) Hydration and sedation - not found to be more beneficial than bed rest
(3) β-adrenergic receptor agonists
=> delay delivery for no more than 48 hours
- Ritodrine (Yutopar)
: Side effects - maternal tachycardia, hypotension, apprehension, chest tightness or pain, pulmonary edema, death, hyperglycemia, hyperinsulinemia, hypokalemia, lactic and ketoacidosis, emesis, headaches, tremor, fever, hallucinations
(4) Magnesium sulfate
- presumably calcium antagonist effect
- 4g loading dose -> continuous infusion of 2g/hr
- dose-dependent toxocity에 유의 => 자세한 것은 PBL했던 것 참조
(5) Prostaglandin inhibitors
; aspirin and other salicylates, indomethacin, naproxen, and sulindac
- adversely affect on fetus => ∴ 널리 이용되지 못함
(closure of the ductus arterio년, necrotizing enterocolitis, intracranial hemorrahge, etc.)
- ritodrine에 비해 48이상으로 문만을 지연시키는 효과가 좋고 materal side effect는 적지만, neonatal morbidity가 증가하는 단점이 있다.
(6) Calcium channel blocker
- ritodrine보다 효과는 더 좋지만 maternal side effect가 심각하다
(Systemic vasodilatory effect -> maternal hypotension -> ↓uteroplacental perfusion)
- enhancing the magnesium toxicity!!
(7) Nitric oxide
- IV nitroglycerin은 magnesuim sulfate와 tocolytic effect 비슷
(8) Combination therapy
- single drug으로 충분하지 않을 때, 시도해 볼 수 있다.