OB pharmacology — magnesium, oxytocics, tocolytics, RhoGAM

◆Magnesium sulfate

• •Preeclampsia with severe features → seizure prophylaxis (IV loading + maintenance)
• •Eclampsia → treat active seizure
• •Neuroprotection in preterm labor < 32 weeks (reduces cerebral palsy)
• •Toxicity: hyporeflexia → respiratory depression → cardiac arrest; antidote = CALCIUM GLUCONATE
• •Monitor: deep tendon reflexes, urine output, respirations, serum Mg

◆Tocolytics (preterm labor)

• •Indomethacin (NSAID): < 32 weeks (closes PDA + oligohydramnios after 32 wks)
• •Nifedipine: any GA; first-line if > 32 wks
• •Terbutaline: short-term only (FDA warning for >48-72 hr — maternal cardiac toxicity); avoid in DM, cardiac disease
• •Magnesium: also tocolytic but mainly for neuroprotection now
• •Don't tocolyze: chorioamnionitis, IUFD, abruption with hemodynamic instability, severe preeclampsia, lethal fetal anomaly
• •Always give: betamethasone (lung maturity) + Mg (if <32 wks) + GBS prophylaxis

◆Postpartum hemorrhage uterotonics

• •Oxytocin: first-line, given IV/IM after delivery
• •Methylergonovine: 2nd line; CONTRAINDICATED in HTN (incl preeclampsia) — vasoconstriction
• •Carboprost (PGF2α): CONTRAINDICATED in asthma — bronchospasm
• •Misoprostol (PGE1): rectal; bronchospasm OK but no methyl/carbo
• •Tranexamic acid: within 3 hr of PPH onset (WOMAN trial)
• •Sequence: bimanual massage → oxytocin → 2nd uterotonic → balloon → uterine artery embolization → hysterectomy

◆Cervical ripening / induction

• •Misoprostol (PGE1, off-label): vaginal/oral; AVOID in prior C-section (uterine rupture)
• •Dinoprostone (PGE2): vaginal insert
• •Mechanical: Foley balloon — safe in prior C-section
• •Oxytocin: titrate; risk of hyperstimulation and rupture

◆Rh isoimmunization (RhoGAM)

• •Rh-negative mother + Rh-positive fetus risk → give anti-D IG (RhoGAM)
• •Timing: 28 weeks routinely; within 72 hr postpartum if baby Rh+; after any antepartum bleeding, ECV, amniocentesis, abortion
• •Kleihauer-Betke for massive fetomaternal hemorrhage → may need higher RhoGAM dose

◆Other key OB drugs

• •Betamethasone: lung maturity, give 24–34 weeks at risk of preterm delivery (also up to 36+6 in late preterm)
• •Indomethacin: closes PDA in newborns; also tocolytic
• •Methotrexate: ectopic pregnancy (unruptured, β-hCG < 5000, no fetal cardiac activity)
• •Mifepristone + misoprostol: medical abortion < 70 days
• •Pre-pregnancy folate 400 µg (4 mg if prior NTD or on AEDs)

OB drug contraindications