The level of physical activity among pregnant urban Ethiopian women was estimated to be low, and according to the World Health Organization classification  the participants would on average be classified as sedentary to lightly active. The low PAL estimated using the objective method was supported by the 24-hour physical activity recall, with large amount of sedentary activities reported.
The incorporation of both physiological and biomechanical signals in the estimation of AEE has several advantages compared to other measures of physical activity (e.g. questionnaire data). The monitor used in the current study was a small, lightweight monitor, it was easy to use in the field, and the method used for converting heart rate and acceleration into AEE provide similar estimates of AEE as indirect calorimetry in both laboratory settings [21, 22] and in the free-living environment, the latter showing good agreement with the doubly labeled water method in adults from Cameroon . Combined heart rate and movement sensing has previously been used in pregnant populations [10, 11] but to our knowledge never among pregnant women from a low-income country. Therefore, the results presented in this paper are unique and novel, with physical activity being assessed objectively in a large sample from an understudied population. Nonetheless, AEE estimates should be interpreted with some caution, since the method is yet to be validated against doubly labeled water in pregnant African women. The main limitation of the study was the cross-sectional design. Inference on changes during pregnancy as suggested by the differences between women of different gestational age of pregnancy in this study should, therefore, be made with caution. In addition, gestational age was assessed by self-report and not verified with ultrasound, so some degree of measurement error would also exist in this variable.
Pivarnik et al. have demonstrated that the relationship between HR and oxygen uptake (VO2) changes during pregnancy and will cause an overestimation of AEE at rest and light activities and an underestimation during moderate and vigorous activities. Given our study population and setting, we chose a self-paced walk test as proxy measure of cardiorespiratory fitness, which we utilized statistically to account for some of the between-individual variance in the HR-VO2 relationship; a step test or similar exercise test using prescribed higher intensity levels would have been too strenuous for most of the participants. Considering that the women in the present study predominantly were engaged in light intensity activities, AEE might be slightly overestimated. However, our calibration approach would remove some of the bias reported using HR-based measures during such activities, and the addition of movement sensing would further minimize this bias.
PAL is based on AEE estimated from movement and heart rate as well as the estimate of resting metabolic rate based on age, sex, height, and weight, derived for use in non-pregnant individuals . Although pregnancy would most likely increase resting metabolic rate [23, 24], the resting metabolic rate estimate may carry a positive bias in Africans ; hence PAL in this study may be estimated with a fair degree of uncertainty.
The overall level of physical activity was low both when reported as AEE (kJ/kg/day) and PAL. Furthermore, the women spent most of their time in activities of sedentary and light intensity and had only a small amount of moderate-to-vigorous intensity activity. The level of physical activity among these pregnant women was also low compared to physical activity among non-pregnant women from Africa [2, 26]. However, these studies are of older date and the ongoing urbanization currently occurring in many low-income countries has changed the traditional physical activity pattern . When comparing physical activity among Ethiopian women in the current study with other studies using the same method, AEE was 20% lower and time engaged in moderate-to-vigorous activity was less than half compared to non-pregnant women from Cameroon  and even lower compared to rural Kenyan women . Furthermore, AEE was 30% lower compared to a group of pregnant women from Sweden  and AEE of the Ethiopian women resembled inactive pregnant women more than active pregnant women from Switzerland . These observations are supported by the fact that energy intake in these pregnant Ethiopian women was also found to be low (6.2 MJ/day, detailed data not shown); hence the low level of physical activity may be an appropriate adaptation to low energy intake to ensure adequate nutrition for the fetus, or may simply be explained by less physically demanding work during pregnancy.
In accordance with previous studies, AEE was negatively associated with gestational age [2, 24, 28, 29] even though the energy cost associated with a specific activity increase continuously during pregnancy . It is therefore likely that women do decrease their activity and likely also the types of activity they engage in with less vertical acceleration during the course of their pregnancy. High fat and low muscle mass in the upper arm were also associated with lower AEE, indicating a low habitual physical activity in women with less favorable body composition, which may have already existed prior to pregnancy or alternatively suggesting that body composition may relate to how a women manage to maintain her activity level when she becomes pregnant.
Cardiorespiratory fitness (sleeping heart rate)
More advanced stage of pregnancy (gestational age) was associated with higher sleeping HR, which is in accordance with previously reported results . The increase in sleeping HR during the course of pregnancy has been proposed to be caused by a decrease in parasympathic influence  and, therefore, may not necessarily reflect lower fitness. However, AFA was positively associated with sleeping HR and since this measure is likely unaffected by pregnancy, AFA may reflect the amount of fat present prior to pregnancy. A higher AFA during pregnancy may therefore reflect a lower cardiorespiratory fitness carried into pregnancy.
Muscular fitness (grip strength)
AMA was positively associated with grip strength. It was expected that the amount of muscle in the upper arm was associated with muscular strength of the forearm. In general, muscular fitness was highest in women with high bodyweight, combined with low amounts of fat and high amounts of muscle. Height was independently positively associated with muscular fitness. Being taller is generally associated with longer limbs and larger hands, which by virtue of longer levers provide better working conditions for the muscles and hence enhanced ability to generate force.
In summary, concerns have been raised that pregnant women in developing countries have high levels of physical activity which can have negative effects on the child. However, our observations indicate that current levels of physical activity among pregnant urban Ethiopian women, when assessed by an objective method, are relatively low and that these women only seldom engage in activity of moderate-to-vigorous intensity. Furthermore, they do not seem to perform a large amount of physically demanding work, such as cleaning and outdoor work. Our study sample, however, only included women residing in urban Ethiopia, whose activity level may not be comparable to pregnant women living in rural areas. Low levels of physical activity have been linked to increased risk of diabetes and cardiovascular diseases in adult Cameroonian [27, 32], and to increased risk of gestational diabetes mellitus as well as preeclampsia in high-income countries [33–35]. However, it is unclear whether or not the low physical activity observed in the present study carry the same increased risks or simply is an appropriate adaptation to a low energy intake. Nonetheless, it is not unlikely that both nutrient intake and physical activity need to increase in these women for optimal health and additional research is required to identify the determinants for such behavior changes. To this end, future research should focus on physical activity among pregnant women in low-income countries, its determinants, and its implications for the health of mother and child.