Infantometer

Infant and child height (or length) were measured by trained Tsimane research assistants using a portable Seca infantometer or stadiometer (Hamburg, Germany). For the purposes of analysis, length and height were treated interchangeably. Weight was measured using a Tanita digital scale to the closest tenth of a kilogram. If infants were too small to stand on the scale, their weight was calculated by subtracting the weight of the mother from the combined weight of the mother holding her infant. Maternal height and weight measures were collected using the same instruments as used for the child, either at the same visit, or within 30 days of the child’s visit. Tsimane-specific z-scores for HAZ, child WHZ and maternal BMI-for-age (BMIZ) were calculated using the open-source R package localGrowth (https://github.com/adblackwell/localgrowth), which utilizes growth curves from [39 (link)]. These growth references better describe intra-population comparison by using locally-relevant growth trajectories [39 (link)]. Child WHZ and maternal BMIZ (in models for children under 3 years) were used as markers of energetic status, to model the effects of interactions between child energetic status and immune function on child HAZ. To determine the child’s age, exact birthdates were verified by the mother or pulled from longitudinal THLHP medical records.

Weight, length, and head circumference (HC) of the infants were measured by trained research assistants within 24 h after birth and then every month (within 1 week after completion of the month) until the age of 6 months. The trained research assistants were tested for interrater reliability to avoid measurement bias. Infant-mother pairs went to hospital or home visited for follow up study.
Infants were weighed naked on a digital weighing scale (Seca 272) to the nearest 1 g. Infantometer (Seca 210) was used to measure the infants’ length to the nearest 1 mm. Head circumference was measured to the nearest millimeter using a non-stretch plastic measuring tape encircling the head at the occipital protuberance level posteriorly and supraorbital ridges anteriorly.
Intrauterine growth status was classified using PediTools Fenton 2013 [26 (link)]. Infants were considered SGA if their birth weight was less than the 10th percentile of the reference. They were classified as AGA if their birth weight was at or above the 10th percentile. The infants were classified as symmetrical SGA when all of the birth parameters (weight, length, and head circumference) were less than the 10th percentile of the reference and asymmetrical SGA if only body weight was less than the 10th percentile of the reference [27 (link)].

Anthropometric and BC measurements were performed for both mothers and infants at 3 months postpartum (3.1 ± 0.1 months; range: 2.9–3.5). Maternal weight was measured using Seca electronic scales (±0.1 kg; Seca, Chino, CA, USA). Height was self-reported by mothers or measured against a marked wall. Infant clothes were removed before measuring weight using electronic scales (Medela Inc.). Infant crown to heel length was measured in supine position with an infantometer (Seca, Chino, CA, USA). Infant head circumference was measured with non-stretch tape to the nearest 0.1 cm. Infant z-scores (WLZ), weight-for-age z-scores (WAZ), length-for-age z-scores (LAZ), BMIAZ and head circumference-for-age z-scores (HCAZ) were measured using the World Health Organization (WHO) Anthro software v3.2.2 [30 ].
Maternal and infant BC was measured with bioelectrical impedance spectroscopy using a battery-operated bioelectrical impedance analyser Impedimed SFB7 (ImpediMed, Brisbane, QLD, Australia) according to the protocols described previously by Gridneva et al. [31 (link)]. In addition to standard BC measurements (fat-free mass (FFM), FM, and %FM), the height-normalized BC indices of mothers and infants (FFMI, FMI) as well as FM to FFM ratio (FM/FFM) were calculated as described previously [6 (link)].