This study examines how three medications commonly used during fetal surgery, fentanyl, rocuronium, and atropine, behave in the fetus. The primary goal is to understand their pharmacokinetics (how the drugs are absorbed, distributed, and cleared), pharmacodynamics (how they affect fetal physiology), and how they transfer between mother and fetus through the placenta. The secondary goal is to measure drug levels in discarded fetal blood samples collected during clinically indicated procedures and relate those levels to fetal heart rate, heart rate variability, movement, gestational age, and fetal size. An optional maternal blood draw component will allow comparison of maternal and fetal drug concentrations to better understand placental transfer. The study does not change clinical care or require extra fetal procedures, and findings may help create safer, evidence-based fetal anesthesia dosing strategies tailored to gestational age.

Inclusion Criteria:

• Pregnant and undergoing maternal-fetal surgery requiring clinically indicated fetal anesthesia and planned fetal blood sampling as part of usual fetal care
• Fetus is with or without congenital heart disease

Exclusion Criteria:

• Patient or fetus have any known liver or kidney disease
• Patient or fetus has any known allergy to fentanyl, rocuronium, or atropine
• Patient has taken inhibitors, inducers or substrates of CYP3A4 other than midazolam (including erythromycin, ranitidine, verapamil, antihistamines, and dextromethorphan) within 24 hours of surgery.

Pregnant women undergoing maternal-fetal surgery