Immunoglobulin M (IgM), Serum
Immunoglobulin M (IgM) is the first antibody produced during an acute immune response. Unlike IgG, IgM does not cross the placenta. Mild reductions during pregnancy primarily reflect physiologic hemodilution rather than impaired immune function.
| Units | Nonpregnant Adult | 1st Trimester | 2nd Trimester | 3rd Trimester |
|---|---|---|---|---|
| mg/dL | 50 – 300 | 1 – 309 | 2 – 290 | 0 – 361 |
| g/L | 0.50 – 3.00 | 0.01 – 3.09 | 0.02 – 2.90 | 0 – 3.61 |
Clinical note: Because IgM does not cross the placenta, fetal IgM production reflects in utero infection (e.g., congenital CMV, toxoplasmosis, rubella).
Causes of low IgM in pregnancy
- Physiologic hemodilution of pregnancy
- Primary immunodeficiency (e.g., selective IgM deficiency)
- Protein loss (nephrotic syndrome, protein-losing enteropathy)
- Severe malnutrition or malabsorption
- Immunosuppressive therapy
- Bone marrow suppression or lymphoproliferative disorders
Causes of high IgM in pregnancy
- Acute infection
- Chronic inflammatory disease
- Autoimmune disorders (e.g., rheumatoid arthritis)
- Liver disease
- Polyclonal hypergammaglobulinemia
- Monoclonal IgM gammopathy (Waldenström macroglobulinemia — rare)
References
- Lockitch G. Handbook of Diagnostic Biochemistry and Hematology in Normal Pregnancy. CRC Press; 1993.
- Abbassi-Ghanavati M, Greer LG, Cunningham FG. Obstet Gynecol. 2009.