1.3 Medication Use in Pregnancy and Lactation
Key Takeaways
- The FDA Pregnancy and Lactation Labeling Rule took effect June 30, 2015 and eliminated the A, B, C, D, and X letter categories, replacing them with narrative subsections 8.1 Pregnancy, 8.2 Lactation, and 8.3 Females and Males of Reproductive Potential; any answer option that cites a letter category is describing an obsolete system.
- Relative infant dose equals the infant dose in mg/kg/day divided by the maternal dose in mg/kg/day, multiplied by 100; a relative infant dose under 10% is conventionally regarded as compatible with breastfeeding for a healthy term infant.
- Drugs cross into milk most readily when they are low in molecular weight, minimally protein bound, lipophilic, and weakly basic, because milk at roughly pH 7.2 is slightly more acidic than plasma at 7.4 and ion-traps weak bases.
- The FDA contraindicated codeine and tramadol in children under 12 years, contraindicated tramadol under 18 after tonsillectomy or adenoidectomy, and advised against both in breastfeeding mothers in April 2017 because CYP2D6 ultrarapid metabolizers generate dangerous morphine concentrations.
- Organogenesis spans roughly weeks 3 through 8 after conception, the window in which structural teratogens such as isotretinoin, valproate, warfarin, and methotrexate cause malformations; angiotensin-converting enzyme inhibitors and angiotensin receptor blockers instead cause second- and third-trimester fetopathy with renal failure and oligohydramnios.
1.3 Medication Use in Pregnancy and Lactation
Every neonate a pediatric pharmacist encounters arrives having already been exposed to whatever their mother took. The BPS Pediatric Pharmacy content outline places pregnancy and lactation inside the Pediatric Patient Characteristics domain for exactly this reason: perinatal pharmacology is a pediatric competency, not an obstetric one. The two recurring consultations are "is it safe to continue this drug while breastfeeding?" and "what does this in-utero exposure mean for the infant in front of me?"
Where the Label Information Lives
The FDA Pregnancy and Lactation Labeling Rule (PLLR), effective June 30, 2015, retired the A / B / C / D / X letter categories and replaced them with narrative subsections 8.1 Pregnancy, 8.2 Lactation, and 8.3 Females and Males of Reproductive Potential. The regulatory architecture of those subsections, and the reasons the letter system failed, are covered in the pediatric drug legislation section of the regulatory chapter. This section takes the label as given and works on the clinical question the label is meant to support: what do you tell this mother, today, about this drug?
[!IMPORTANT] A BCPPS answer option that says "this drug is Pregnancy Category C" is describing a system the FDA retired. Over-the-counter drug labels were never subject to the letter system at all.
Why Some Drugs Concentrate in Milk
Transfer into milk is passive diffusion driven by a concentration gradient across the mammary alveolar epithelium. Four physicochemical properties govern it:
FACTORS INCREASING TRANSFER INTO HUMAN MILK
Low molecular weight ...... < 200 Da diffuses through aqueous pores;
> 800-1,000 Da (heparins, insulin,
monoclonal antibodies) essentially excluded
Low protein binding ....... only the FREE fraction can diffuse
(warfarin is >99% bound -> compatible)
High lipophilicity ........ partitions into the milk fat phase
Weak base ................. milk pH ~7.2 < plasma pH ~7.4, so weak
bases ionize and are TRAPPED in milk
Long half-life ............ accumulates with repeated maternal doses
Two derived parameters quantify the result:
- Milk-to-plasma (M/P) ratio — the concentration in milk divided by that in maternal plasma. An M/P above 1 signals active concentration in milk, but the ratio alone is misleading: a drug with a high M/P but a tiny maternal dose may still deliver a trivial amount to the infant.
- Relative infant dose (RID) — the parameter that actually matters:
Convention: an RID below 10% is generally considered compatible with breastfeeding a healthy term infant. The threshold tightens for preterm infants, neonates in the first weeks of life, and infants with renal or hepatic impairment, because their clearance of whatever does transfer is immature.
Worked example. A mother takes sertraline 100 mg/day and weighs 70 kg, so her dose is 1.43 mg/kg/day. The infant weighs 4 kg and ingests an estimated 0.02 mg/kg/day from milk.
Well under 10% — consistent with sertraline's standing as a preferred antidepressant during lactation.
The Reference to Use
LactMed, the National Library of Medicine's peer-reviewed Drugs and Lactation Database, is free, updated monthly, and reports RID, M/P, and documented infant effects with citations. It is the appropriate first-line source and outranks a package insert's boilerplate "a decision should be made whether to discontinue nursing."
Drug-by-Drug Guidance During Lactation
| Category | Representative agents | Assessment |
|---|---|---|
| Preferred | Acetaminophen, ibuprofen, sertraline, paroxetine, most penicillins and cephalosporins, heparins, insulin, levothyroxine, most inhaled and topical agents | Low RID or negligible oral bioavailability in the infant |
| Use with monitoring | Fluoxetine (long half-life active metabolite; watch for infant irritability and poor feeding), metronidazole (bitter taste, theoretical concern with single 2 g dose), fluconazole (high RID but well tolerated), benzodiazepines (monitor sedation, prefer short-acting and lowest effective dose) | Feasible with an alert caregiver and a defined monitoring plan |
| Avoid / contraindicated | Cytotoxic chemotherapy, radiopharmaceuticals (time-limited interruption with pumping and discarding), amiodarone (very long half-life plus iodine load causing neonatal thyroid dysfunction), codeine and tramadol, retinoids, and ongoing substance misuse | Risk of significant infant exposure or toxicity |
The Codeine and Tramadol Rule
In April 2017 the FDA acted on reports of respiratory depression and death in children:
- Codeine is contraindicated for pain and for cough in children under 12 years.
- Tramadol is contraindicated in children under 12 years, and additionally under 18 years for post-operative pain after tonsillectomy or adenoidectomy.
- Both are not recommended in breastfeeding mothers because a mother who is a CYP2D6 ultrarapid metabolizer converts codeine to morphine (or tramadol to O-desmethyltramadol) at accelerated rates, and the resulting milk concentrations have caused fatal infant respiratory depression.
The alternative for maternal post-partum pain is scheduled acetaminophen and ibuprofen, escalating to a short course of a non-CYP2D6-dependent opioid such as morphine or hydromorphone at the lowest effective dose when needed.
Practical Counseling Levers
- Time the dose. Taking a short-half-life drug immediately after a feed maximizes the interval before the next feed and lets the maternal peak pass.
- Choose the less-transferred member of a class. Within the same therapeutic class, prefer the agent with higher protein binding, shorter half-life, and no active metabolites.
- "Pump and dump" is rarely required and never for drugs with a low RID. It is appropriate for radiopharmaceuticals and defined chemotherapy washout intervals, and it must be paired with instructions to maintain supply.
- Never stop breastfeeding reflexively. The documented benefits of human milk — reduced necrotizing enterocolitis, otitis media, gastroenteritis, and sudden infant death — are large, and unnecessary weaning is itself a harm.
In-Utero Exposure: What the Pediatric Pharmacist Inherits
Organogenesis spans roughly weeks 3 through 8 post-conception, the window for structural malformation. Exposures before implantation tend to follow an all-or-nothing pattern; exposures in the second and third trimesters cause functional or growth effects rather than structural defects.
| Exposure | Effect | Timing |
|---|---|---|
| Isotretinoin | Craniofacial, cardiac, thymic, and CNS malformations; extremely high risk | Organogenesis; iPLEDGE program mandates two contraceptive methods and monthly pregnancy testing |
| Valproate | Neural tube defects (roughly 1% to 2%), facial dysmorphism, and a documented reduction in childhood IQ with autism-spectrum risk | Neural tube closes by week 4; cognitive effects extend across gestation |
| Warfarin | Fetal warfarin syndrome — nasal hypoplasia, stippled epiphyses | Highest risk weeks 6 to 9; CNS bleeding risk later |
| ACE inhibitors / ARBs | Fetopathy: renal tubular dysgenesis, anuria, oligohydramnios, skull hypoplasia, limb contractures | Second and third trimesters (not a classic first-trimester teratogen) |
| Methotrexate | Aminopterin-like syndrome, craniofacial and limb defects, pregnancy loss | Organogenesis |
| Tetracyclines | Permanent tooth discoloration and enamel hypoplasia | After about week 20, when mineralization begins; the same reason tetracyclines were long avoided under age 8 |
| Lithium | Ebstein anomaly of the tricuspid valve; absolute risk is low but above baseline | First trimester |
| SSRIs (late gestation) | Poor neonatal adaptation syndrome — jitteriness, tachypnea, feeding difficulty, usually self-limited over days | Third trimester |
| Antenatal corticosteroids | Beneficial, not harmful: betamethasone or dexamethasone given to the mother at 24 to 34 weeks accelerates fetal lung maturation and reduces respiratory distress syndrome, intraventricular hemorrhage, and necrotizing enterocolitis | 24–34 weeks |
Preconception folic acid remains the single highest-yield intervention: 400 mcg daily for all people capable of pregnancy, rising to 4 mg daily for those with a prior neural-tube-defect pregnancy or on valproate or carbamazepine.
Neonatal Opioid Withdrawal Syndrome
In-utero opioid exposure produces neonatal opioid withdrawal syndrome (NOWS), presenting at 24 to 72 hours for short-acting opioids and as late as 5 to 7 days for methadone.
- First-line care is non-pharmacologic, captured by the Eat, Sleep, Console model: rooming-in, skin-to-skin contact, low stimulation, and breastfeeding where not contraindicated. This approach has reduced pharmacotherapy use and length of stay compared with scoring-driven escalation using the Finnegan tool alone.
- Pharmacotherapy, when the infant cannot eat, sleep, or be consoled, is oral morphine or methadone, weaned by protocol. Buprenorphine is an increasingly used alternative.
- Phenobarbital or clonidine are adjuncts, particularly for polysubstance exposure.
- Maternal methadone or buprenorphine treatment is not a contraindication to breastfeeding; both have low relative infant doses, and breastfeeding reduces NOWS severity.
Practice Pearls & BCPPS Exam Traps
- Exam Trap 1: A high milk-to-plasma ratio does not by itself contraindicate breastfeeding. Calculate the relative infant dose; that is the parameter that scales with what the infant actually receives.
- Exam Trap 2: Large-molecule drugs — insulin, heparins, and monoclonal antibodies such as infliximab — do not meaningfully enter milk, and what little does is destroyed in the infant gut. They are generally compatible with breastfeeding.
- Exam Trap 3: ACE inhibitors cause fetopathy in the second and third trimesters, not first-trimester structural teratogenesis. Options that place ACE inhibitor risk in organogenesis are testing that distinction.
- Board Rule: When asked about maternal post-partum analgesia in a breastfeeding mother, the correct answer never contains codeine or tramadol.
A breastfeeding mother of a healthy 3-week-old term infant is discharged after a vaginal delivery with a perineal laceration and asks about pain control. Her obstetric team has proposed codeine/acetaminophen. Which recommendation from the pediatric clinical pharmacist is correct?
A pediatric clinical pharmacist is consulted about a mother taking a maintenance medication while breastfeeding her 2-month-old. Which set of physicochemical properties predicts the GREATEST transfer of drug into human milk?
A neonate is admitted to the newborn nursery at 38 weeks gestation. The maternal history documents chronic hypertension treated with lisinopril continued through the second and third trimesters. Which clinical consequence should the pediatric clinical pharmacist anticipate and monitor for?