To the best of our knowledge, this is the first study to examine parental height and BMI associations with ultrasound measures of fetal growth in a rural Indian population with a high prevalence of low birth weight. Our results suggest that associations between paternal height and fetal head circumference and femur length are detectible earlier in gestation (17–29 weeks) compared to the same associations with maternal height. Fetuses of mothers with a higher BMI had smaller head circumferences at 17 weeks, but head growth caught up to become larger in late gestation. Paternal BMI showed no significant associations with any component of fetal growth. Maternal but not paternal BMI, was positively related to placental size at 17 weeks.
Our study has several important strengths. First, we used serial fetal biometry to characterise prenatal growth as opposed to birth weight. Second, ultrasound measurements were made by only 2 sonographers and intra and inter-observer reliability was excellent. Third, we were able to obtain accurate LMP dates since health workers visited women every month prior to pregnancy. Fourth, the maternal BMI data are likely to be an accurate representation of pre-pregnant BMI as data were collected within a 3 month window prior to pregnancy. Fifth, a sensitivity analysis showed that non-paternity was unlikely to have affected our main findings. And last, the use of conditional growth variables as a model of fetal growth allowed an exploration of the onset and pattern of parent–fetal associations.
The main limitation is a lack of power, a bigger study may have exposed some of the trends in the timing of associations over gestation that we reported, however intergenerational data with fetal ultrasound from populations in rural India are rare. We were also restricted to investigating growth intervals at relatively fixed time points. However, while more densely spaced measures may have offered a deeper insight, it does increase signal-noise ratio in the growth variable.
Maternal height was positively related to conditional growth in the last gestational interval (29 weeks — birth) for head circumference and femur/birth length. A study of American singletons also found positive associations between maternal height and head circumference at 31 weeks, and with femur length at 25 weeks 
. Paternal height was positively related to conditional head circumference growth from 17 to 29 weeks and size of the femur at 29 weeks. The associations with father's height were thus evident earlier in gestation compared to the associations with maternal height. In an Australian study, paternal height was positively related to fetal femur length at 24 weeks, while maternal height was negatively related 
. We could find no other studies to compare this differential pattern.
Positive correlations of maternal and paternal height with newborn skeletal measurements like length and head circumference are well described 
. Our new finding that correlations between paternal height and fetal size and growth occurred earlier in gestation compared to associations with maternal height, may be related to the strong correlation between paternal height, but not maternal height, and placental volume at 17 weeks gestation. It suggests that paternally inherited genes influencing skeletal growth are expressed throughout gestation, while those from the mother are expressed in late gestation, and may be related to larger placental size. These differences could be mediated by epigenetic mechanisms; it is known that a number of genes that promote fetal growth are paternally imprinted [28,29]
. There is insufficient data like ours to know if the associations observed are specific to our population or seen in other populations. Taken together, the maternal and paternal findings would be consistent with the ‘selfish gene’ theory, which suggests that paternally inherited genes promote fetal growth, regardless of maternal nutritional status 
For maternal BMI, there was a positive association with HC growth in the 29 week to birth interval, consistent with findings from Goldenberg et al. (1993) 
, who found positive associations with HC at 31, 36 weeks and birth. The associations with maternal BMI at 17 weeks were negative for all fetal components in our study. In particular, in this population with a high prevalence of chronic maternal undernutrition, women with lower BMI had fetuses with larger head circumference at 17 weeks. A comparison of our finding with the American study 
is not possible because the coefficients were not reported, however, Blake et al. 
did report negative associations between femur length and both maternal and paternal BMI, although the evidence was weak. It is unlikely that our results are due to gestational dating errors related to maternal BMI, as there was no relationship between the discrepancy in ultrasound and LMP dated gestational age and maternal BMI (p = 0.6), unlike that found in another study 
. Pre-pregnancy BMI is a marker of energy balance, nutrition and adiposity. Interestingly, maternal BMI was positively associated with placental volume at 17 weeks. An explanation for this differing placental–fetal response could be that better nutrition (higher maternal BMI) stimulates placental development at the expense of the fetus in early pregnancy, possibly as a strategy to enable greater fetal growth in late gestation. A larger placenta, whilst requiring more energy itself, has a larger surface area, which allows increased nutrient transfer to the fetus.
The different patterning of associations reflected by the mother and father's BMI on placental volume and the suggestion of a different pattern of associations on the fetal components — particularly head circumference, suggest some direct and potentially modifiable effects of maternal nutritional status on placental and fetal growth. Higher maternal BMI may promote placental growth in early-mid pregnancy, and this may enhance fetal growth in late pregnancy, although this interpretation of our findings is speculative. We do not yet know the implications of variations in placental and fetal growth for future health in the child. Recent studies in the Helsinki birth cohort suggest that placental size and shape at birth, newborn size, and ratios of placental to newborn size, predict adult hypertension, and that these associations are conditioned by maternal nutritional status 
. In further follow-up of this cohort, we will be able to explore associations of placental measurements and fetal growth patterns with cardio-metabolic risk factors in the children.
We recommend that epidemiological studies exploit the use of ultrasound to characterise variations in fetal and placental growth, and their timing during gestation. Such measures will allow more carefully designed analyses into questions related to the developmental origins of disease, and offer insight beyond that possible with birth weight alone.