7.3 Perinatal Pharmacotherapy: Pregnancy and Lactation

Maternal Pharmacokinetics and Physiological Adaptations

Pregnancy triggers profound anatomical and endocrine changes designed to support fetal growth. These physiological adaptations alter drug absorption, distribution, metabolism, and elimination across all three trimesters.

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|             MATERNAL PHARMACOKINETIC (PK) ADAPTATIONS                   |
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|  ABSORPTION:                                                            |
|  - Elevated progesterone slows gastric emptying and GI transit time     |
|  - Increased gastric pH (decreased gastric acid secretion)              |
|  - Increased pulmonary blood flow -> enhanced inhaled drug absorption   |
|                                                                         |
|  DISTRIBUTION:                                                          |
|  - Plasma volume expands by 40-50% (plasma +1.5 L)                      |
|  - Total body water expands by 6-8 L; maternal adipose +3-4 kg          |
|  - Expanded Vd for both hydrophilic and lipophilic drugs                |
|  - Serum albumin falls (hypoalbuminemia of pregnancy) -> higher free    |
|    active fraction of protein-bound drugs (phenytoin, valproate)        |
|                                                                         |
|  METABOLISM:                                                            |
|  - INDUCED ENZYMES (Increased Clearance): CYP3A4, CYP2D6, CYP2C9, UGT   |
|    -> Lamotrigine glucuronidation (UGT1A4) surges up to 300% ->         |
|       drastic drop in serum level; seizures occur if dose not increased |
|  - INHIBITED ENZYMES (Decreased Clearance): CYP1A2, CYP2C19             |
|    -> Prolonged half-life of caffeine, theophylline                     |
|                                                                         |
|  EXCRETION:                                                             |
|  - Renal blood flow and GFR increase by 50-80% starting in 1st trimester|
|  - Dramatically accelerated clearance of renally eliminated drugs       |
|    (lithium, penicillins, cephalosporins, enoxaparin, digoxin)          |
|  - Frequently requires HIGHER doses and SHORTER dosing intervals        |
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Clinical Impact on Medication Dosing

• Lamotrigine Dosing Surge: Estrogen powerfully induces UGT1A4, the primary enzyme responsible for lamotrigine glucuronidation. Serum lamotrigine concentrations fall by $50-70%$ as early as the first trimester, precipitating loss of seizure control. Frequent therapeutic drug monitoring and dosage increases of up to $200-300%$ of pre-pregnancy baselines are required, followed by immediate postpartum dose tapering to avoid postpartum toxicity as estrogen levels plunge.
• Accelerated Renal Clearance: Renally excreted drugs (e.g., ampicillin, cefazolin, low molecular weight heparins, lithium) have significantly shortened elimination half-lives and lower trough concentrations, necessitating increased daily doses or more frequent administration (e.g., every 8 hours instead of every 12 hours).

Placental Drug Transfer and Teratogenic Mechanisms

Determinants of Placental Transfer

The human placenta is a lipid membrane separating maternal and fetal circulations. The vast majority of medications cross the placenta by passive diffusion:

1. Molecular Weight ($MW$): Drugs with $MW < 500\text{ Da}$ cross the placenta readily (most conventional small-molecule drugs). Drugs with $MW 500-1000\text{ Da}$ cross more slowly. Large macromolecules with $MW > 1000\text{ Da}$ (such as insulin, heparin, certolizumab pegol) do not cross the placenta in clinically significant amounts.
2. Lipid Solubility: Highly lipophilic medications (e.g., general anesthetics, opioids, benzodiazepines) rapidly traverse the syncytiotrophoblast.
3. Ionization: Non-ionized drugs cross more rapidly. Fetal blood is slightly more acidic ($\text{pH } 7.30-7.35$) than maternal blood ($\text{pH } 7.40$), causing weak bases to become ionized in fetal circulation, leading to ion trapping and higher fetal drug accumulation.
4. Protein Binding: Only the unbound (free) fraction crosses the placenta.

Critical Timing of Teratogenic Exposure

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|                   CRITICAL PERIODS OF GESTATIONAL EXPOSURE              |
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|                                                                         |
|  1. PRE-DIFFERENTIATION / PRE-IMPLANTATION (Fertilization to Day 14):   |
|     - 'ALL-OR-NONE' Period                                              |
|     - Severe toxic insults destroy the blastocyst (spontaneous abortion)|
|     - Non-lethal insults are repaired by totipotent cells (normal baby) |
|     - Major structural anatomical malformations do NOT occur            |
|                                                                         |
|  2. EMBRYONIC PERIOD / ORGANOGENESIS (Weeks 3 to 8 post-conception;     |
|     Gestational Weeks 5 to 10):                                         |
|     - CRITICAL WINDOW FOR MAJOR STRUCTURAL TERATOGENESIS                |
|     - Organs undergo primary morphogenesis:                             |
|       * Neural Tube: Days 18-28 (Folic acid critical!)                  |
|       * Heart: Days 20-40                                               |
|       * Limbs: Days 24-36                                               |
|       * Palate & Facial Structures: Days 40-60                          |
|     - Exposure to teratogens produces gross anatomical defects          |
|                                                                         |
|  3. FETAL PERIOD (Week 9 to Delivery):                                  |
|     - Organs are formed; undergo growth, differentiation, & maturation   |
|     - Teratogenic insults cause functional deficits, growth restriction,|
|       CNS behavioral alterations, and fetotoxicity (e.g., ACEi fetopathy|
|       or NSAID-induced ductus arteriosus constriction)                  |
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High-Yield Teratogens and Evidence-Based Canadian Management

Major Known Teratogens Matrix

Clinical Management of Chronic Conditions in Pregnancy (SOGC Guidelines)

• Hypertension in Pregnancy (Chronic or Gestational):
  • First-Line Oral Agents: Labetalol (combined $\alpha/\beta$-blocker; $100-400\text{ mg}$ PO BID-TID), Methyldopa (central $\alpha_2$-agonist; $250-500\text{ mg}$ PO BID-QID), and Long-Acting Nifedipine (dihydropyridine CCB; $20-60\text{ mg}$ daily XL).
  • Severe Acute Hypertension ($\text{BP} \ge 160/110\text{ mmHg}$): Labetalol IV, Hydralazine IV, or Nifedipine intermediate-release PO.
  • Strictly Contraindicated: ACE inhibitors, ARBs, and Direct Renin Inhibitors.
• Diabetes in Pregnancy (Gestational and Pregestational):
  • Insulin is the gold standard (does not cross the placenta; NPH, Regular, Detemir, Glargine, Lispro, Aspart). Metformin and glyburide cross the placenta and are second-line alternatives when insulin is refused or unmanageable.
• Venous Thromboembolism (VTE) & Anticoagulation:
  • Low Molecular Weight Heparin (LMWH, e.g., Enoxaparin, Dalteparin) is the standard of care (does not cross placenta, predictable PK, lower heparin-induced thrombocytopenia and bone loss risks). Unfractionated heparin is used near term. DOACs and warfarin are avoided.
• Nausea and Vomiting of Pregnancy (NVP):
  • First-Line: Doxylamine succinate $10\text{ mg}$ / Pyridoxine hydrochloride $10\text{ mg}$ (Diclectin, up to 4 tablets daily per SOGC guidelines).
  • Second-Line: Dimenhydrinate ($50-100\text{ mg}$ PO/PR q4-6h), Promethazine, Metoclopramide, or Ondansetron.
• Folic Acid Supplementation for Neural Tube Defect (NTD) Prevention:
  • Low-Risk Women: $0.4\text{ mg}$ ($400\text{ }\mu\text{g}$) PO daily starting at least 2 to 3 months prior to conception throughout pregnancy and postpartum.
  • Moderate Risk (Diabetes, obesity, GI malabsorption, family history): $1.0\text{ mg}$ PO daily.
  • High Risk (Personal history of prior NTD pregnancy, or taking valproate/carbamazepine): $4.0-5.0\text{ mg}$ PO daily pre-conception until 12 weeks gestation, then $0.4-1.0\text{ mg}$ daily throughout pregnancy.