What magnesium preparation is safe for pregnant women with constipation?

Magnesium Hydroxide is the Safest Magnesium Preparation for Pregnant Women with Constipation

Magnesium hydroxide (milk of magnesia) at 400-500 mg daily is the recommended magnesium preparation for pregnant women with constipation, though it should be used with caution in those with renal insufficiency. 1, 2

Why Magnesium Hydroxide Specifically

The 2023 AGA-ACG guidelines explicitly identify magnesium oxide (which converts to magnesium hydroxide in the stomach) as an appropriate osmotic laxative option, with the specific caveat to "use with caution in patients with renal insufficiency and in pregnancy." 1 This is the only magnesium formulation that has been studied in randomized controlled trials for constipation treatment. 1

Other magnesium formulations (citrate, glycinate, lactate, malate, sulfate) have unknown bioavailability and clinical efficacy for constipation in pregnancy. 1

Treatment Algorithm for Constipation in Pregnancy

First-Line: Dietary Modifications

• Increase fiber intake to 30 g/day through fruits, vegetables, whole grains, and legumes 2
• Ensure adequate fluid intake, particularly water 2
• Allow sufficient time for bowel movements without straining 2

Second-Line: Bulk-Forming Agents (if ineffective after 1 week)

• Psyllium husk or methylcellulose are safe due to minimal systemic absorption 2
• Soluble fiber (psyllium) is preferred over insoluble fiber as it improves both stool viscosity and transit time 2

Third-Line: Osmotic Laxatives (if ineffective after 1-2 weeks)

Preferred options in order:

1. Lactulose 15 g daily - This is the only osmotic agent specifically studied in pregnancy 1, 2

   • May cause maternal bloating and flatulence, which can be dose-limiting 1, 2
   • Titrate based on symptom response 1

2. Polyethylene glycol (PEG) 17 g daily - Considered safe in pregnancy 2, 3

   • A 2024 Japanese study found PEG alleviates constipation without inducing diarrhea in pregnant women 3
   • No significant difference in perinatal prognosis or side effects compared to magnesium oxide 3

3. Magnesium hydroxide 400-500 mg daily - Can be used but requires caution 1, 2

   • Must avoid in women with creatinine clearance <20 mg/dL 1
   • Hypermagnesemia risk increases with renal impairment as systemic regulation depends on renal excretion 1
   • Prior studies used higher doses (1,000-1,500 mg daily), but lower doses are typically used in clinical practice 1

Critical Safety Considerations

Renal Function Assessment is Mandatory

Before prescribing magnesium hydroxide, check renal function - systemic magnesium regulation is maintained by renal excretion, making hypermagnesemia more likely with significant renal impairment. 1 Avoid magnesium supplements entirely if creatinine clearance is <20 mg/dL. 1

Avoid Stimulant Laxatives

Do not use stimulant laxatives (senna, bisacodyl) during pregnancy due to conflicting safety data. 2 These should be avoided throughout pregnancy despite being commonly available over-the-counter. 2

Why Lactulose May Be the Better Choice

Despite the question asking specifically about magnesium, lactulose deserves strong consideration as it is the only osmotic agent with pregnancy-specific safety data. 1 The 2023 AGA-ACG guidelines note this distinction explicitly in their treatment tables. 1

A 2023 systematic review found lactulose was associated with 267 more responders per 1,000 patients (defined as >1 spontaneous bowel movement from baseline or lack of need for other laxatives) compared to placebo. 1

Common Pitfalls to Avoid

• Do not assume all magnesium formulations are equivalent - only magnesium oxide has been studied in RCTs for constipation 1
• Do not prescribe magnesium without checking renal function first - hypermagnesemia can occur with impaired clearance 1
• Do not use the high doses from research studies (1,000-1,500 mg daily) as initial therapy - start with 400-500 mg daily 1
• Do not overlook lactulose as the preferred osmotic laxative - it has the best pregnancy safety profile among osmotic agents 1, 2