Lactation is associated with decreased areal bone mineral density (aBMD). Replenishment occurs especially after ceased lactation. Changes in volumetric bone mineral density (vBMD), microstructure, and dimensional parameters are unknown and may clarify the role of lactation for skeletal health.

The objective of the study was to test the hypothesis that lactation is associated with changes in aBMD, vBMD, microstructure, and dimensional parameters.

At baseline (0.5 mo after delivery) and 4, 12, and 18 months thereafter, bone was assessed using dual-energy x-ray absorptiometry and high-resolution peripheral quantitative computed tomography.

Eighty-one fair-skinned postpartum women and 21 controls aged 25–40 years were recruited. The completion ratio was 73%. Postpartum women were categorized depending on duration of lactation: 0–3.9, 4–8.9, and 9 months or longer.

During the first 4 months, aBMD decreased at several sites (geometric mean ± SE; −0.73% ± 0.21% to −3.98% ± 0.76%) in women lactating at least 4 months. During the same time, cortical vBMD at the ultradistal tibia decreased in women lactating 4–8.9 months (−0.26% ± 0.08%) and 9 months or longer (−0.49% ± 0.10%). At 12 months postpartum, cortical thickness (≥9 mo, −2.48% ± 0.41%) and trabecular thickness (4–8.9 mo, −2.14% ± 0.92%; ≥ 9 mo, −2.56% ± 1.21%) also were lower than baseline. No decreases were found in women lactating less than 4 months or in controls in these parameters. At 18 months postpartum, both cortical vBMD (≥9 mo, −0.77% ± 0.17%) and trabecular thickness (4–8.9 mo, −2.25% ± 1.25%; ≥ 9 mo, −3.21% ± 1.41%) were lower in women with long lactation.

Decreases in cortical vBMD, thickness, and trabecular thickness at the ultradistal tibia were found in women lactating 4 months or longer. Longer follow-up is needed to confirm whether women with extended lactation recover fully or whether the changes could potentially lead to an increased risk of fracture in later life.