• asymmetric dimethylarginine
• fetal growth restriction
• feto-placental circulation
• intra-partum hypoxia
• labor
• nitric oxide
• nitroso-hemoglobin
• peri-natal adaptation
• vascular endometrium

Background
Endothelium-derived nitric oxide (NO) is a highly reactive inorganic free radical with widespread biological actions, including vascular regulation, neurotransmission, hormone secretion and inflammation.
Potential roles for NO in the human uterus include vasodilatation (both before implantation, and in the uteroplacental and systemic circulation during pregnancy).
Nitric oxide may also be involved in diseases of pregnancy, from unexplained infertility and recurrent miscarriage and/or defective placentation in the first period of gestation. During the third trimester of pregnancy, a change in the NO production may be involved in pre-eclampsia and fetal growth restriction (FGR). NO may also have a compensatory function, and several situations in the adult document this possibility.
This thesis aimed to verify whether the NO activation represents a unifying mechanism for the preservation of blood delivery to the fetus during different adverse conditions.

The working hypothesis was that endothelial cells from feto-placental vessels are key determinants of any situation in which the fetus is exposed to a hypoxic insult and requires an adequate amount of oxygen for well-being, including chronic hypoxemia leading to FGR and transitory hypoxiemia during normal or disturbed labor.
Further we aimed to define the nature of any linkage between NO and FGR and, coincidentally, provide a possible insight into the alleged negative impact of FGR on adult health.

Methods
The approach was comprehensive and included: (i) measurement of NO and its main metabolite, nitrite (NO2), along with the natural NO synthase inhibitor asymmetric dimethylarginine (ADMA); (ii) analysis of Doppler velocimetry in umbilical arteries; and (iii) assessment of gene profile in umbilical vein endothelial cells (HUVEC) collected at the time of delivery.
Study comprised term pregnancies with average (n =40) or small-for-gestational age body weight (n = 20) (both scheduled for umbilical Doppler velocimetry at 36 wk), pregnancies with isolated preterm FGR (n = 15) and bi-chorial, bi-amniotic twin pregnancies with discordant fetal growth (n = 12).
Cord blood (artery and vein) was collected in all cases, while peripheral blood (heel sampling) was obtained from certain newborns at the time of delivery and 24 or 72 h afterwards.
A second analysis was performed on the sub-group of pregnancies where delivery of a normal newborn had occurred at term, either vaginally or through cesarean section without/with prior labor (n = 20). In addition, cases from the same cohort but not considered earlier, where delivery had been complicated by fetal hypoxia (n = 15), were examined. Then, collective values were cross-analyzed depending on the mode of delivery and the presence/absence of fetal hypoxia. Separately, the same variables were measured over the first three days of life in term neonates from vaginal and cesarean (with/without labor) delivery uncomplicated by hypoxia.

Results
The NO system within the placenta and the fetus itself may be important in maintaining a suitable oxygenation of the offspring through any intervening hypoxic insult.
Compensatory feto-placental up-regulation of the NO system during FGR was shown. In detail, umbilical blood nitrite (p < 0.001) and S-nitrosohemoglobin (p = 0.02) rose with fetal growth restriction while asymmetric dimethylarginine decreased (p = 0.003). Nitrite rise coincided with an abnormal Doppler profile from umbilical arteries. Our analysis of the expression of a wide set of endothelial genes suggests that this phenomenon is part of a re-setting of endothelial function, as an adaptative event to sustain placental blood flow. In fact, in the presence of fetal growth restriction, umbilical vein endothelial cells produced more nitrite and also exhibited reciprocal changes in vasodilator (upwards) and vasoconstrictor (downwards) transcripts.
Moreover, elevation in blood nitrite and S-nitrosohemoglobin persisted post-natally in the fetal growth restriction offspring, potentially modifying the endothelial phenotype and possibly representing an element of risk for cardiovascular disease in adult life.
Similarly to FGR, where NO may be of use to counteract chronic impairment in oxygen inflow to the fetus, fetal and placental NO may also be important to facilitate blood flow to the fetus during labor and delivery.

In particular, active labor was associated with higher NO and NOHb concentrations in the umbilical vein blood. Accordingly, HUVEC from labor-based deliveries presented greater eNOS expression and activity. The same blood variables, however, presented an opposite trend in the umbilical artery. Further activation of the NO system occurred with deliveries complicated by offspring hypoxia along with a fall of ADMA levels.
This set of responses may help adapt to post-natal breathing, extending the potential protective role of NO to the peri-partum period; in fact, the upward change in NO activity progressed over the first 24 hrs after birth to subside by 72 hrs.

Conclusions
The role of NO in diverse uterine conditions may have great clinical implications in developing therapeutic strategies to prevent NO-related disorders. Indeed, if a role for NO is confirmed, pharmacological modification of NO activity may lead to novel therapeutic applications. Moreover, the NO system within the placenta and the fetus itself may be important in maintaining a suitable oxygenation of the offspring through any intervening hypoxic insult. FGR is typified by increased nitric oxide production during pregnancy and after birth. This response is viewed as an adaptive event to sustain placental blood flow. However, the phenotypic characteristics of endothelial cells linked to the synthesis of NO might contribute some sort of imprinting to the vulnerable newborn determining functional vascular changes that may be important for postnatal adaptation but that may as well be long-lasting, possibly programming the infant in the long-term.
As the NO system is a key player in preserving fetal oxygen availability in chronic conditions of increased demand, transitory hypoxemia during delivery is associated with enhanced NO function in the feto-placental district with a concomitant greater utilization of the agent by the fetus. This normal event is magnified with intra-partum hypoxia and, in all cases, persists in the immediate post-natal period. We regard this set of changes as a protective mechanism whose finality is to maintain an adequate oxygenation of the fetus through delivery with an attendant smooth transition from intra- to extra-uterine life.