Vitamin D (25-hydroxyvitamin D) reflects total body vitamin D stores and is the preferred test for evaluating vitamin D status in pregnancy. Physiologic hemodilution and altered vitamin D metabolism lead to trimester-specific changes.
| Units | Non-pregnant Adult | 1st Trimester | 2nd Trimester | 3rd Trimester |
|---|---|---|---|---|
| ng/mL | 14 – 80 | 18 – 27 | 10 – 22 | 10 – 18 |
| nmol/L | 35 – 200 | 45 – 67 | 25 – 55 | 25 – 45 |
Causes of High 25-Hydroxyvitamin D
- Excess supplementation
- Granulomatous diseases (sarcoidosis, TB) — ↑ extrarenal 1α-hydroxylase activity
- Hyperparathyroidism
- Vitamin D intoxication
- Lab artifact due to lipemia
Causes of Low 25-Hydroxyvitamin D
- Insufficient sunlight exposure
- Low dietary intake
- Obesity (vitamin D sequestration in adipose tissue)
- Malabsorption syndromes (celiac, Crohn disease, pancreatic insufficiency)
- Chronic liver disease (↓ 25-hydroxylation)
- Chronic kidney disease
- Anticonvulsant therapy (increased catabolism)
- Dark skin pigmentation (lower cutaneous synthesis)
References
- Abbassi-Ghanavati M, Greer LG, Cunningham FG. Pregnancy and laboratory studies: a reference table for clinicians. Obstet Gynecol. 2009;114(6):1326–31. PMID: 19935037