The purpose of this study was to examine the possible relationship between birth weight, birth history, and developmental milestone achievement in a retrospective, convenience sample from a clinic population identified as or suspected of having sensory processing disorder. Our results showed that infants with related birth complications who had lower birth weights than infants without birth complications were NBW, AGA, or SGA. Ages of achievement for developmental milestones and birth weight were linked with birth weight and gestational age considered with birth weight category. Earlier ages for independent sitting in HBW infants, eating solids in NBW infants, and walking onsets in LBW and NBW infants were associated with larger birth weights. Rolling at a younger age in LGA infants, earlier walking and speaking words in AGA infants, and sooner independent sitting in SGA and AGA infants were also linked with higher birth weights.
Our results showed that birth history was associated with lower birth weights for AGA, NBW, and SGA infants. We hypothesized that birth history would be linked with lower birth weight for SGA infants because of medical complications (e.g. respiratory distress) requiring professionals to deliver infants expediently. Lower birth weight may have been related to birth history for AGA and NBW infants in our sample because these infants also had complications necessitating a swift delivery that required assistance (e.g. forceps). Also, our sample consisted of children who were being evaluated for sensory processing disorder, so there may have been a higher incidence of birth complications in this sample even for AGA and NBW infants [16
We found that larger birth weights were associated with earlier onset ages for some developmental milestones. However, the relationship between birth weight and milestone achievement varied according to birth weight category and gestational age classification considered with birth weight. For gestational age classification considered with birth weight, we found that heavier SGA and AGA infants sat earlier and that heavier AGA infants walked and said words earlier. Previous studies have shown that SGA infants tend to have less body fat than LGA infants and some AGA infants [17
], and that LGA infants have lower lean body mass than AGA infants [18
]. Less body fat in SGA infants and more lean body mass in AGA infants may have allowed them to achieve some motor milestones sooner than the LGA infants in our sample. One of the most difficult tasks for infants is to hold their bodies up against gravity due to uneven body proportions [19
]. Until 24 months old, infants and toddlers are top heavy, which makes their center of mass high [19
]. During forward progression, a high center of mass requires toddlers to work hard at mitigating the forward movement their center of mass [23
]. When older children and adults walk, they use active plantarflexion of the stance limb to propel themselves upward. Therefore, they keep the acceleration of their center of mass positive when each foot contacts the ground [23
]. But, toddlers lack adequate muscle strength and coordination to control the forward movement of their center of mass when they walk. Instead, at foot contact, the acceleration of new walkers’ center of mass is negative, which means that they fall downward into their steps [23
]. Therefore, LGA infants may be at a disadvantage for sitting and walking because of high fat and low muscle mass, but at an advantage for rolling because it does not require holding themselves up against gravity. Although we do not have body composition data for these infants, it could be that the HBW and NBW infants in our sample have body composition characteristics (e.g. high muscle mass) that allowed them to achieve earlier sitting and walking. Also, children with sensory processing disorder commonly have low muscle tone, poor postural control and stability, and decreased vestibular functioning [27
]. Thus, early motor skills development may be impacted by these factors as well. Increased weight may be an advantage for them, but too much may result in poor milestone development because of the above factors. It is important to further examine the relation of sensory processing disorder to these areas.
Mastering postural control sooner with independent sitting and walking may predispose children to achieving other developmental milestones sooner; we may have found a relationship between birth weight and eating for NBW infants as well as saying words to AGA infants because their ability to be upright (i.e. earlier walking for NBW and earlier sitting and walking for AGA infants) allows for eating solids earlier and for engaging in interactions with others sooner. Meeting motor milestones earlier has been shown to foster meeting other developmental milestones earlier. For instance, the beginning of crawling and walking are associated with increases in verbal [28
] and cognitive abilities [29
]. The findings about relationships between birth weight and the onset of developmental milestones from this study is important to our understanding of children with sensory processing disorder. In particular, it highlights the need for examining the additive influence of sensory processing disorder in light of children’s development [16
The limitations to this study are that: 1) the retrospective nature of this study did not allow us to control how many participants were represented in categories based on sex, birth weight category, gestational age classification considered with birth weight, the presence of incomplete histories, or what parents’ exact definition of the onset of milestones were and 2) the fact that we sampled our data from a clinical population may limit the generalizability of our findings, but provides important information on describing this population of children with sensory processing disorder. Future studies involving the collection of prospective data from non-clinical sources are underway to ameliorate these limitations. This study is a first step into examining how variations in birth weight and gestational age considered with birth weight categories relate to birth history and developmental milestone achievement.