Interpretation
Decreased levels of inflammatory markers in cord blood and neonatal serum after CS compared with VD has been described before (31, 32), but most previous studies did not stratify for the type of CS (pre-labour or in-labour). A few studies regarding CRP have been described, that found similar differences in new-born levels depending on birth form as we describe (33, 34). In a study among teenagers, whose mothers had entered the active phase of labour before CS, spontaneous and toll-like receptor‐stimulated cytokine release was increased, compared to controls born by pre-labour CS (35). This concurs with another study showing that the risk for early childhood infections are higher in children born by pre-labour CS compared to children born by in-labour CS or VD (1, 36). One of the common explanations for the immunological differences after CS and VD has been microbial transmission from mother to child, either by transmission during vaginal birth or by microbial invasion of the amniotic cavity after PROM (32, 36). However, in our study, the exclusion of all cases with PROM did not make any difference in the significance of the biomarker levels. Following national guidelines, prophylactic antibiotics are given during or after both pre- and in-labour CS in Denmark (37), and the hospital stay afterwards are the same (in contrast to after VD where the woman and infant often leaves the hospital a few hours after birth). The explanation for the observed differences in our study is thus more likely to be the influences of stress hormones and/or the physical pressure from the labour process. During VD, cortisol and other stress hormones increase. Elevated cortisol at birth is a known indicator of hypothalamic-pituitary-adrenal axis activation, which is important for regulation of stress and many other body processes (38). The observed sustained inflammatory response 2-4 days after birth in our study after in-labour CS or VD may thus be protective for the foetus later in life, and could possible even explain some of the increased risk for autoimmune and inflammatory disorders after pre-labour CS.
The intracellular inducible HSP70 are one of the major HSPs involved in numerous cellular functions, such as cytoprotection, anti-apoptosis, and immune regulatory effects (39). Increased temperature, exposure to oxidative stress, such as hypoxia, viral infection, and ischemia-reperfusion injury, can induce the expression (39). The fact that HSP70 had similar infant levels after in-labour CS and VD may indicate that the stress effect is the same provided the labour have been initiated, regardless if the delivery ends with CS or VD.
Animal studies have shown that mouse brains have increased contents of norepinephrine, dopamine, serotonin and metabolites of dopamine and serotonin after vaginal deliveries compared to mice delivered by CS. The turnover ratios of the neurotransmitters were also higher in the mouse brains after vaginal delivery, and the later adult mice showed different behavioural patterns (40). We have recently found significantly lower neonatal levels of BDNF in new-borns later diagnosed with autism spectrum disorders (41). In the current study, we did not see any correlation with the neurotrophic markers BDNF, NT-3, and S100B, and the delivery form. This might indicate that neurodevelopment are not dependent on delivery form.
VEGF is a growth factor that stimulates vasculogenesis and angiogenesis after stress, and it is an essential factor for placental development (42). VEGF is expressed at sites of injury and inhibits the activity of nitric oxide synthase, preventing inflammation (43), and it is present at high levels in the central nervous system (44). VEGF is a potent stimulator of angiogenesis in asthma (45), and both VEGF and EGF are stimulators of mucins in the respiratory tract, which concentrations have been reported as positively correlated with asthma disease severity (46, 47). We are not aware of any other studies regarding delivery forms and growth factors. The increased levels after pre-labour CS could indicate that the mechanisms for the increased risk for asthma have been initiated already 2-4 days after birth, but this needs to be more thoroughly investigated in further studies using asthma as an outcome.
Higher concentrations of CRP in neonatal males compared to females has been described before (48). This may be due to hormonal differences, as estradiol (which is highest in females) decreases the production of CRP (49, 50), and increases the gene expression of BDNF (51). Although these early-life differences may simply be an epiphenomenon, it might be the precursors to later in life higher frequencies of different disorders in males, but this needs further investigation. Circumcision is rarely performed in Denmark (52), thus this cannot explain any of the gender biomarker differences (52).
The overall increasing levels of inflammation biomarkers from GA 37-42 concurs with a study showing that the leucocyte count increases with GA up to week 40 (53). The levels of HSP70 decreases until week 40, but then increases in week 41 and 42. This may support the ongoing discussion about earlier deliveries of foetuses after GA week 40 due to increased risk for the overdue foetus (54). The growth factors and S100B decreased for each week of GA, while the neurotrophic factors did not depend on GA. Low GA at birth is a well-known risk factor for later cognitive impairment (55). This effect appears to persist even when born at term, and a study has reported that there are significantly more children receiving special education the lower the gestational age they were born, even up to GA week 39 compared to week 40 (56). We did not see any differences in the neurotrophic markers that could explain this, but the higher levels of growth factors and S100B could indicate a less mature body and brain.