In this study we report evidence of relatively reduced MCA resistance, MCA PSV and umbilical venous flow rates in male fetuses when compared to females. Sex specific variations in these Doppler indices have not been previously reported. We observed statistically significant differences in the MCA PI, MCA PSV, Umbilical venous flow velocity, and Umbilical venous flow/kg. These differences remained statistically significant even when infants considered small for gestational age (birth weight <10th centile) were excluded from the analysis. No sex specific differences were observed in Umbilical artery PI, Uterine artery PI, the cerebro-umbilical or cerebro-uterine ratios.
Previous authors have suggested male fetuses are more likely to have placental insufficiency. This may be responsible for the increased rates of spontaneous miscarriage, in utero fetal demise, fetal growth restriction, and fetal distress in labour seen in pregnancies with a male fetus 
. Edwards et al (2000) reported higher rates of severe placental dysfunction in male fetuses, evidenced by absent or reversed end diastolic flow in the umbilical artery in a cohort of growth restricted fetuses 
. Our results suggest that subtle differences in the flow characteristics of the Middle cerebral artery exist between male and female fetuses, even in the presence of apparently normal placental function (as evidence by appropriate growth, normal umbilical and uterine artery Doppler resistance indices). Increased cerebral blood flow is a protective mechanism employed by the fetus during periods of hypoxia and is associated with fetal growth restriction 
. The observation of a reduction in Middle Cerebral artery resistance in male fetuses when compared to female counterparts may represent a subtle adaptation to differing levels of placental function. Cerebral redistribution in small for gestational age fetuses has been shown to result in a higher risk of subsequent neuro-developmental deficits at two years of age 
. Several neuro-developmental deficits such as language delay 
and autistic spectrum disorders 
are also known to have a male sex bias. Whilst data from the current study is purely observational, it does warrant further investigation of sex-specific changes in fetal haemodynamics. The differences between male and female fetuses observed here were subtle, but our study population included only appropriately grown foetuses. The differences in cerebral perfusion between the sexes may be more pronounced in cases of placental dysfunction, and could in part, explain the higher risk of poor neuro-developmental outcomes seen in males in pregnancies complicated by fetal growth restriction.
The cerebro-umbilical ratio has been suggested as the most accurate method of identifying a pathological brain sparing circulation 
in growth restricted fetuses. However, we did not observe a significant difference in the cerebro-umbilical ratio between male and female fetuses. This is unsurprising considering this study focused on appropriately grown infants, unlikely to have significantly perturbed placental function.
Our results also demonstrate a reduced Umbilical venous flow rate in male fetuses when compared to female fetuses, in the absence of a difference in Umbilical artery Doppler resistance indices. The difference in flow rate was due only to reduced flow velocities in male fetuses, as there was no difference in the size of the Umbilical vein between the different sexes. Other authors have reported reduced Umbilical venous flow velocity in cases of fetal growth restriction and speculated that these changes occur prior to those in the Umbilical artery 
. Our results suggest lower umbilical venous flow rates/kg for term fetuses than were previously reported by Rigano et al 
. This discrepancy is due to the use of different formulae to calculate flow in a vessel. When calculating flow within a cylindrical vessel it is important to adjust for the differing flow rates within that vessel, with flow at the centre of the vessel occurring at a greater velocity than flow at its perimeter (due to friction with the vessel walls). This is corrected for by the multiplication factor of 0.5 in our formula. The paper by Rigano et al does not include this adjustment. Without this adjustment our figures for umbilical venous flow/kg are very similar to those previously published. Our results demonstrate a reduction in umbilical venous flow rate in male infants, without evidence of a similar change in the Umbilical artery.
Sex specific differences have also been observed in the fetal response to a sub-optimal intra-uterine environment. In pregnancies complicated by untreated maternal Asthma, female fetuses have reduced birth weight, whereas male fetal birth weight is unaffected 
. Furthermore, sex specific alterations in placental genes involved with growth and inflammation have also been observed in cases of maternal hypoxia suggesting that aberrations in placental functions can occur in a sex specific manner 
. The identification of sex specific differences in Middle Cerebral artery and Umbilical venous flow characteristics in this study necessitates further investigation to establish the physiological significance of this finding.
All ultrasound scans in this study were performed by a single trained clinician, limiting inter-observer variability. The fetal sex was unknown at the time of the ultrasound, thus limiting bias. Potential confounding factors such as ethnicity, maternal age and BMI, and the gestation the ultrasound was performed at were compared between the male and female cohorts. No significant difference was observed between maternal age, BMI, and percentage of primiparous women in each group. Ethnicity showed an excess of women of “other” ethnicity in the female fetus cohort, but otherwise the groups were not statistically different.
Findings from this study of sex specific variations in Middle cerebral artery Doppler indices and umbilical venous flow have not been previously reported. Although the clinical application of results from this study may be limited at the present time, they do provide further insight into gender specific circulatory differences in the fetal period. Our findings are also intriguing, particularly as more recent data suggest that although these cerebral perfusion differences do not persist into the neonatal period 
,there is evidence that as adults, women have higher cerebral blood flow compared to men