Objectives: The overall aim of this thesis was to evaluate the impact of maternal body mass index (BMI) on obstetric and neonatal outcomes in adolescents as well as on treatment with oxytocin infusion during labor regarding cumulative oxytocin dose and plasma levels of oxytocin.

Material and Methods: Studies I and II were nationwide Swedish register studies including 31,386 primiparous adolescents subdivided in BMI groups and obesity classes according to early pregnancy BMI. Additionally, 178,844 low-risk normal weight adult women, called “the standard women” were included for comparison with the adolescents in the different BMI groups.

Study III was an observational study conducted at seven maternity wards in the southeast region of Sweden, including 1,097 nulliparous women with spontaneous start of labor receiving oxytocin infusion during labor. The study group was subdivided according to BMI on admission to the labor ward. Cumulative oxytocin dose was registered from the start of the oxytocin infusion until the neonate was born. Study IV was an observational pilot study conducted at the maternity ward in Linköping, including 40 women in term labor receiving oxytocin infusion during the first stage of labor. The women were subdivided into obese and non-obese categories according to BMI on admission to the labor ward. Serial peripheral venous blood samples were taken during oxytocin infusion in the first stage of labor. The plasma oxytocin concentrations were analyzed with ultra performance liquid chromatography - Orbitrap tandem mass spectrometry at the Department of Chemistry Biomedical Centre, Uppsala University.

Results: Compared with their normal weight counterparts and compared with the standard women, overweight and obese adolescents had increased risks for adverse pregnancy and neonatal outcomes such as preeclampsia, stillbirth, post-term pregnancy, neonates large for gestational age and neonates with low Apgar score. In labor, compared with the normal weight adolescents, the overweight and obese adolescents had a decreased chance for a normal vaginal delivery (VD), increased risk for Cesarean section (CS) and postpartum hemorrhage (PPH). However, compared with the standard women, the overweight adolescents had increased chance for a normal VD, and decreased risk for instrumental VD, CS, obstetric anal sphincter injury (OASI) and PPH in VD. The obese adolescents had an increased risk for CS but a decreased risk for instrumental VD and OASI and they had the same chance for a normal VD and no increased risk for PPH in VD.

The mean cumulative oxytocin dose increased in the BMI groups (normal weight 2278 [2748] mU, overweight 3108 [3839] mU and obese 4082 [4895] mU (p<0.0001)) and the mean maximum oxytocin infusion rate during the first stage of labor was higher in the obese group compared with the overweight group of women (15.5 [9.5] mU/min vs. 13.6 [9.1] mU/min (p<0.05)). When adjusting for birth weight, epidural analgesia, and cervical dilation at the start of oxytocin infusion, the statistically significant differences were no longer seen. The obese women had increased median levels of oxytocin in plasma at an oxytocin infusion rate of 3.3 mU/min compared with the non-obese women (19.55 ng/mL [IQR 10.61; 29.06 ng/mL] vs. 6.97 ng/mL [IQR 5.55; 13.70 ng/mL], p=0.016).

Conclusions: There are several adverse obstetric and neonatal outcomes associated with overweight and obesity in adolescents. It is important for clinicians to be aware of these outcomes in maternal health care. Nevertheless, overweight adolescents seemed to have better labor outcomes compared with low-risk adults, which is useful knowledge when risk-assessments are made in the labor ward.

In spontaneous onset of labor, the cumulative oxytocin dose and maximum oxytocin infusion rate were higher in women with obesity compared with lower BMI groups. One explanatory factor seems to be that women with obesity received the oxytocin infusion at an earlier cervical dilatation stage. Furthermore, obese women seemed to have higher levels of oxytocin in plasma at the beginning of the oxytocin infusion. The oxytocin-oxytocin receptor complex must be further investigated to gain more knowledge on how to optimize the treatment of obese women with oxytocin infusion during labor.