Bone Mineral Density and Microarchitecture Changes During Pregnancy

Pregnant woman
Pregnant African American mother holding her stomach
There are changes in maternal bone mineral density and microarchitecture during pregnancy that exceed the natural age-related changes in women.

During pregnancy, there are changes in maternal bone mineral density (BMD), microarchitecture, geometry, and distribution that exceed the natural age-related changes in women, according to study results published in the Journal of Bone and Mineral Research.

Although reproduction can have a significant effect on BMD and bone architecture, there are conflicting data on pregnancy-induced bone loss in existing literature. Currently, pregnancy and lactation are not considered risk factors for osteoporosis, but both can significantly alter the skeletal structure.

The goal of the current study was to quantify pregnancy-induced changes in compartmental BMD (trabecular and cortical volumetric BMD) and bone mineral architecture using peripheral quantitative computed tomography (pQCT) and high-resolution pQCT techniques between the second (14 to 16 weeks) and third (34 to 36 weeks) trimesters.

The study included healthy women aged 30 to 45 with a singleton pregnancy that was achieved by natural conception. The participants were recruited between March 2017 and December 2018 at the Rosie Hospital Cambridge University Hospitals in Cambridge, United Kingdom.

The researchers’ primary hypothesis was that in pregnant women, there would be decreases in trabecular volumetric BMD measured by pQCT and high-resolution pQCT and changes in trabecular microarchitecture as measured by high-resolution pQCT. Secondary hypotheses were that there would be lower cortical thickness and density in addition to endosteal resorption at the proximal radius and tibia by pQCT and distal radius and tibia by high-resolution pQCT.

The cohort included 53 pregnant women and 37 nonpregnant, nonlactating women (mean age, 35.4 years) with available pQCT and high-resolution pQCT scans from the tibia and radius at the early second trimester and third trimester of pregnancy or a similar scan interval for the control group. Multiple linear regression models were used to assess group differences (expressed as standard deviation scores [SDS] ± the standard error of the mean) in changes between baseline and follow-up.

Compartment-specific changes in volumetric BMD were found with both high-resolution pQCT (cortical volumetric BMD) and pQCT (trabecular volumetric BMD).

Although there were decreases in volumetric BMD in both groups, pQCT at the distal tibia showed that the decrease in total volumetric BMD was 0.65±0.22 SDS greater in pregnant women than in control individuals (P <.01). Pregnant women also experienced a decrease in trabecular volumetric BMD that was 0.50±0.23 SDS greater (P <.05) than that in control individuals. In contrast, no significant differences were noted between groups at the cortical-rich 38% proximal tibia site. In pregnant women, total and cortical volumetric BMD at the distal tibia decreased relative to the control group by 0.49±0.24 SDS (P =.04) and 0.67±0.23 SDS (P <.01), respectively.

Pregnancy-related changes in bone microarchitecture also significantly exceeded the changes in the control group with regard to trabecular number (0.47±0.23 SDS), trabecular separation (-0.54±0.24 SDS), cortical thickness (-1.01±0.21 SDS), and cortical perimeter (0.78±0.23 SDS).

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At the distal radius, no significant difference in any high-resolution pQCT or pQCT bone outcome was found between the groups. Cortical volumetric BMD and endosteal circumference increased by 0.50±0.22 SDS and 0.46±0.23 SDS (both P <.05), respectively, compared with controls, whereas cortical thickness decreased by -0.50±0.23 SDS (P <.05).

The study had several limitations, including lack of in vivo bone densitometry data from pregnant women and no data on potential changes in bone outcomes postpartum or during lactation.

“Although the magnitude of these pregnancy-related changes in the appendicular skeleton are small, if they reflect global changes across the skeleton at large, they may contribute substantially to the [calcium] requirements of the fetus,” noted the researchers.

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Breasail MO, Prentice A, Ward K. Pregnancy-related bone mineral and microarchitecture changes in women aged 30 to 45 years [published online March 2, 2020]. J Bone Miner Res. doi:10.1002/jbmr.3998