Objectives: To clarify the predisposing factors associated with blood loss after delivery in mothers with full-term singleton babies.

Methods: In this retrospective cohort study, we investigated 1,294 women who delivered singleton babies vaginally in 2011 at a medical center in Tokyo. We determined the amount of blood loss after delivery and covariates of age, parity, pre pregnancy body mass index (BMI), gestational weight gain (GWG), gestational week, pregnancy complications, lifestyles of smoking and drinking, placental weight, and infant weight and sex.

Results: The majority of participants had lost less than 500 ml of blood (n=868, 67%), 21% lost between 500–799 ml of blood (n=273), 12% lost 800 ml or more of blood (n=153). The amount of blood loss statistically increased (p<0.001) as pre pregnancy BMI category level increased from underweight (<18.5 kg/m²), normal (18.5–22.9 kg/m²), to overweight/obesity (≥23 kg/m²). Compared with the least category of GWG <8.2 kg, ≥8.2 kg GWG was statistically associated with a larger amount of blood loss category (p=0.032). Multinomial logistic regression analyses demonstrated that with the reference pre pregnancy BMI 18.5–22.9 kg/m², obese and obesity mothers with pre pregnancy BMI ≥23 kg/m² were at an increased risk of blood loss [OR 2.28, 95%confidence interval (95%CI): 1.48–3.50 for the category of 500–799 ml and OR 2.15, 95%CI: 1.29–3.59 for a category of 800 ml≤)]. In addition, pregnancy induced hypertension (PIH) (p=0.010) and infant weight (p<0.0001) significantly increased the risk of blood loss.

Conclusions: In mothers with full-term singleton babies, increased pre pregnancy BMI overweight/obesity, PIH, and infant weight, were suggested to be risk factors for increased amount of blood loss.