Maternal nutritional status is considered an important risk factor for the progression of labor and birth weight. Poor birth outcomes, such as experiencing a prolonged labor or being small for gestational age, represent a risk factor for maternal as well as infant morbidity and mortality. Understanding the factors that may affect maternal nutritional status is necessary. The objective of the present study was to examine the association between the practice of breastfeeding during late pregnancy and the risk of having a prolonged active phase of labor (PAPL) or giving birth to a small for gestational age (SGA) baby.;A case-control study was conducted with two matched controls for each case of SGA or PAPL. Between March 2006 and April 2007 three Peruvian hospitals were visited daily. Mothers who met the inclusion criteria were interviewed and information regarding maternal as well as newborn characteristics was extracted from their clinical charts. Hair samples for zinc analysis were collected on a subsample of mothers. The association between the overlap of breastfeeding during late pregnancy and the risk of SGA or PAPL was tested with conditional logistic regression.;No differences were found in the proportion of mothers who breastfed during late pregnancy between the cases of SGA or PAPL and their respective controls. In addition, there was no difference in maternal hair zinc concentration between cases and controls.;Our findings do not support the hypothesis that the practice of breastfeeding during late pregnancy represents a risk for having a small for gestational age infant or experiencing a prolonged active phase of labor. However, research to confirm these results is needed since negative birth and postnatal consequences have been previously associated with this practice.