A. General principles.

1. Maternal physiologic adaptation to pregnancy.

a. Cardiovascular.

i. Increased: cardiac output, blood volume.

ii. Decreased: peripheral vascular resistance.

b. Respiratory.

i. Increased: tidal volume and respiratory rate.

ii. Decreased: total lung capacity and functional residual capacity.

iii. No change: pulmonary artery pressure.

c. Hematologic.

i. Increased: blood volume, pH, coagulability.

ii. Decreased: hematocrit.

d. Renal.

i. Increased: renal artery perfusion, glomerular filtration rate (GFR), creatinine clearance, renal clearance of medications, and risk of urinary tract infection (UTI).

ii. Decreased: blood urea nitrogen (BUN), serum creatinine, and serum uric acid.

e. Gastrointesinal.

i. Increased: gastroesophageal reflux and risk of aspiration with intubation

ii. Decreased: motility and LES pressure.

2. Diagnostic radiation exposure.

a. Ultrasound and MRI are preferred to imaging that involves ionizing radiation.

b. There is a small risk of carcinogenesis in ionizing radiation at all gestational ages.

c. After the first 14 days, radiation exposure over 0.5 Gy may be associated with an increased risk of congenital malformations, growth restriction, and intellectual disability.

d. Abdominal/pelvic shielding should be used when possible.

e. Single-exposure films minimize radiation/risk to the fetus.

f. Computed tomography (CT) delivers radiation that is likely too low in dose to cause significant teratogenesis, however, should be avoided due to increased risk of carcinogenesis.

3. Medications and pregnancy.

a. Analgesics.

i. Short courses of opiates are tolerated.

ii. Codeine is teratogenic in the first 12 weeks.

b. Nonsteroidal antiinflammatory drugs (NSAIDs): are generally avoided due to possible premature closure of the ductus arteriosus. Short courses can be given in the second trimester to treat pain but are contraindicated in the third trimester.

c. Antibiotics.

i. No known fetal effect: penicillins, cephalosporins, erythromycin, clindamycin, and vancomycin.

ii. Streptomycin and kanamycin: fetal ototoxicity.

iii. Gentamicin can be used if necessary; monitor levels closely.

iv. Sulfonamides should be avoided in third trimester—associated with kernicterus.

v. Tetracycline is teratogenic.

vi. Fluoroquinolones are contraindicated throughout pregnancy due to effects on cartilage development.

d. Anticoagulants.

i. Coumadin: teratogenic in first trimester, later it carries risk of fetal bleeding.

ii. Heparin: does not cross placenta and is the anticoagulant of choice.

iii. Low molecular weight heparin: safe, change to unfractionated in third trimester—low molecular weight heparin associated with epidural hematoma with regional anesthesia.

e. Antihypertensives.

i. Avoid angiotensin-converting enzyme (ACE) inhibitors and angiotensin receptor blockers—associated with fetal renal dysfunction and oligohydramnios.

ii. Sodium nitroprusside can lead to fetal thiocyanide poisoning.

iii. Hydralazine and labetalol are the first line IV medications in pregnancy.

iv. Any patient with elevated blood pressure should be evaluated for preeclampsia by sending a 24-hour urine collection, checking platelets ALT and AST (see section II below).

f. Pressors.

i. Because ephedrine, which has alpha- and beta-stimulating effects, tends to preserve uterine blood flow while reversing systemic hypotension, it may be the preferred pressor to try first. Phenylephrine has been used alone and in combination with ephedrine to reverse maternal hypotension with epidural anesthesia.

ii. Be aware that predominantly alpha-adrenergic agents improve maternal blood pressure but decrease uterine blood flow due to uterine artery vasoconstriction.

g. Tocolytics.

i. Multiple classes: β-adrenergic, NSAID, calcium channel blockers, magnesium sulfate.