Background Infants with intrauterine growth restriction (IUGR) often have metabolic and electrolyte abnormalities. Our aim was to determine the incidence of hypophosphatemia in IUGR versus appropriate for gestational age (AGA) premature infants.

Methods A retrospective review of infants ≤32 weeks or ≤1500 grams who had a serum phosphorus within 48 hours after birth. We collected maternal and neonatal demographic data and electrolyte values. Infants below the 10th percentile on the Fenton Growth Curve were categorized as IUGR. Serum hypophosphatemia was defined as < 4mg/dL and serum hypokalemia as < 3.5mg/dL.

Results Over a 4 year period, 304 infants were eligible. Of these, 54 were IUGR (mean birth weight (BW) of 848 grams and mean gestational age (GA) of 28+6 weeks) and 250 were AGA (mean BW of 1067 grams and mean GA of 27+6 weeks). 48% of the IUGR infants had hypophosphatemia compared with only 6% of the AGA infants (p<0.05). The IUGR infants with hypophosphatemia had a lower birth weight and GA than the IUGR infants without hypophosphatemia. This difference was not observed among AGA infants. 15.1% of the IUGR infants (8/53) had a serum potassium of < 3.5mg/dL compared to 7.6% of the AGA infants (19/250). There was a moderate correlation between serum phosphorus and serum potassium. Overall mortality was < 1%.

Conclusions Hypophosphatemia is very common among IUGR infants < 32 weeks GA and there is a moderate correlation with hypokalemia. These electrolyte abnormalities probably reflect adaptive mechanisms associated with growth restriction in utero.