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First confirmation of the hypothesis that polyhydramnios causes bone maldevelopment

We have read the article entitled ‘Foetal biometry in polyhydramnios: does femur length fall behind?’ (Ipek et al. 2016), in this article, the authors state that chronic polyhydramnios is associated with a decrease in femur length percentiles in the third trimester relative to the second trimester of gestation. This decrease in femur length is not associated with skeletal dysplasia. Intrauterine bone development is influenced by endocrine trophic factors, mechanic stress because of bone loading and hydrostatic pressure. In the first part of gestation, the foetus is fully immersed in the amniotic fluid and it is in the condition similar to a neutral buoyancy. After the foetus has outgrown the intrauterine cavity in the last trimester of gestation, it has 60–80% of its actual weight (Sekuli c et al. 2005). Polyhydramnios increases the degree of the buoyant forces to which the foetus is exposed and consequently decreases the level of mechanical stress on bones. The femur is exposed to mechanical stress during leg and whole body movements. It was hypothesised that the prolonged unloading of the femur because of polyhydramnios could cause its hypoplasia and a decreased mineralisation (Sekulic et al. 2010). A decrease in the femur length percentiles in the third trimester relative to the second trimester of gestation (Ipek 2016) is the first confirmation of the proposed hypothesis. The calcaneus, tibia, femur and vertebrae are also exposed to bone loading (Sekuli c et al. 2005). The same mechanism which affects the size of the femur could affect the size of all weight bearing bones. Additionally, a decreased mechanical stress on bone in polyhydramnios could be accompanied with a decreased bone mineralisation, leading to an increased incidence of bone fracture in the postnatal period. In future prospective studies, it is necessary to obtain exact data about bone condition after birth (peripheral quantitative computed tomography, ultrasound examination, bone turnover markers). The data on muscle condition in polyhydramnios in humans are missing. There are ethical issues involved in obtaining muscle tissue for analysis.