The principal findings of this study are two-fold: firstly, self-selected walking speed has strong significant relationship with net walking energy expenditure during pregnancy; and secondly, the relative mass gain, rather than the absolute mass is a primary factor associated with changes in gait mechanics which may lead to increase in walking energy expenditure.
In this study, although the differences between trimesters in gait kinematics and walking energy expenditure were not statistically significant, there were significant associations between gait kinematics and walking energy expenditure. Similarly to previous studies, self-selected walking speed during third trimester was lower than the first or second trimesters [2, 9], although in this study this decrease was not statistically significant. Changes between trimesters in gross and net walking energy expenditures were also not significantly different. However, self-selected walking speed showed strong significant relationship with net walking energy rate (ml/kg/min) and walking economy (ml/kg/m) (Figs. 1 and 2, respectively), while there was a lack of association with the gross energy expenditure. Since gross energy expenditure contains REE, the variability in REE which is associated with physiological changes due to foetal development, would not be related to the energy expenditure required for walking. Increase in resting energy expenditure (kcal/day) (Table 1) is associated with the increase in mass (Table 2), although the lack of statistical differences may be attributed to the large variability in mass gain between the participants, or the differences in self-reported pre-pregnancy weight.
The relationship of speed of walking and net energy expenditure is largely determined by the COGv (r = 0.70, p ≤ 0.001; r = 0.45, p ≤ 0.01, respectively) (Table 2). Given that the motion of the COG may be regarded as the summation of all forces that act on the body, the significant portion of the total metabolic cost during walking should be attributed to the work required to move the COG [5, 6], especially as the weight of the body increases as in pregnancy. This effect has been demonstrated in our previous article . This relationship indicates that the ability to increase walking efficiency is related to the principle of conservation of mechanical energy during walking that is maximized at certain speeds [4, 5, 25], which participants in this study did not reach. The average self-selected walking speed of 1.08 ± 0.11 m/s did not significantly change during pregnancy and falls within previously reported range from 0.83 m/s  to 1.5 m/s .
While the changes in walking speed were associated with the absolute mass (r = − 0.43, p ≤ 0.01), gait parameters associated with the greater stability during walking, step width and the time spent in double-support stage, were associated with the relative mass gain (r = 0.38, p ≤ 0.01 and r = 0.34, p ≤ 0.05, respectively). Due to weight distribution during pregnancy, the trunk moment of inertia increases leading to need for greater stability . More stability during walking may be obtained by increasing double-support time, increase the step width, or both, in order to create a larger base of support. In addition, lower walking speeds results in an increased double support time, which gives pregnant women more time to react and control additional balance demands during walking [9, 20, 26].
However, these gait changes may result in mechanically inefficient gait which may lead to increase in total energy expenditure . Walking with the bigger base of support results in large side-to-side excursions of the centre of gravity (COG) , which may increase the energy demand as discussed earlier. The results in this study show 13.6% increase of medio-lateral excursions of centre of gravity (COGML) between the first and second trimester, and 39.3% between second and third trimester (p ≤ 0.001). These changes were significantly related to relative mass gain (r = 0.50, p ≤ 0.001), rather than the absolute mass. In late pregnancy, due to large mass gain, width of the pelvic girdle also increases in order to accommodate the growing foetus, which also leads to the increase in the width of the base of support  and consequently the increase in the step width during pregnancy .
While changes in gait mechanics may have a significant impact on walking energy expenditure, the metabolic cost of walking may not be sufficient to alter the overall net energy balance. The increase in absolute REE between the trimesters (although not statistically significant) was largely associated with the mass (r = 0.86, p ≤ 0.001), however, once normalized for the mass REE decreased between subsequent trimesters and showed strong negative correlation with the mass (r = − 0.85, p ≤ 0.001), which is suggestive of energy conservation process during pregnancy associated with the changes in metabolism . However, the difference in REE between the 1st and 3rd trimester was 1.8 kcal/kg, indicating that energy sparing process in a woman with approximate weight of 65 kg (average pre-pregnancy weight in this study = 64.4 ± 14.7 kg), would conserve 117 kcal/day – only a 6.5 to 5.9% increase from 1800 to 2000 kcal/day recommended daily caloric intake for healthy women of the same group and activity level as reported in this study. Considering the relationship of walking speed and net energy expenditure in this study, and the decrease from 1st to 3rd trimesters in walking speed, the difference in energy expenditure conservation by means of walking would equal to 0.5 kcal/min for the same individual. Therefore, for conservation of energy from changes in gait to have a meaningful impact on overall energy expenditure during pregnancy, women would have to walk continually for several hours.
The small impact changes in gait mechanics have on total energy expenditure, allows for gait mechanics to be altered for reasons such as balance or comfort, which may lead to mechanically inefficient gait , but without the significant impact on overall energy expenditure, which helps maintain overall net positive energy balance during pregnancy. Because pregnancy is characterized by the bearing of an extra and “valuable” load, and as such walking efficiency has to be combined with safety. While the additional burden of the growing fetus may increase the demand of mechanical energy, women tend to adopt a strategy that helps them maintain the rate of energy expenditure at a level that can be sustained for a relatively long time. This is also a strategy adopted by individuals who walk with a pathological condition . Considering that the pre-pregnancy physical and physiological characteristics differ among the women studied, this is also the most likely source of large inter-subject variability in gait parameters during pregnancy reported across all similar studies.
The results of this study have to be considered in regard to the limitations presented during data collection. Firstly, not all the pre-pregnancy weight was obtained from participants’ records and was therefore self-reported, which is known to be under-estimated at the times. Secondly, large withdrawal rates prevented longitudinal tracking, which would allow identification of the most common changes occurring during pregnancy in the parameters investigated.