Aims: In many countries, pregnant women’s breastfeeding of the older baby is common and may lead to oxytocin release, uterine contraction and increased maternal energy need. The probable consequences of lactation during pregnancy on the pregnancy outcome and newborn are still unclear. The aim of this study was to investigate the effect of the breastfeeding the older baby during the pregnancy on the pregnancy outcome and newborns’ birth sizes.

Methods: In this case-control study, during 9 months in maternity units of Birjand, of 2308 multiparous pregnant mothers, 320 were selected through sequential sampling method. The case group consisted of 80 mothers who were breastfeeding the older child during their current pregnancy. Control group included 240 mothers homogenized with the case group, who stopped breastfeeding to the old baby at least 3 months before pregnancy. Two clinical trained nurses gathered newborns birth weight, height, head circumference and pregnancy outcome of two groups. Data were analyzed with SPSS 16 software, student T, Pearson correlation, chi square, Mann-Whitney, and Fisher’s exact tests.

Results: There was no significant difference between the pregnancy outcome in the case and control groups (p=0.36), and between the pregnancy outcome and mothers’ breastfeeding behaviors in case group (p=0.69). The mean of newborns’ birth weight (p=0.45), height (p=0.15), and head circumference (p=0.31) in two groups had no significant difference.

Conclusion: Breastfeeding of healthy pregnant mothers to their older child has no remarkable effect on the pregnancy outcome, newborn’s birth weight, height and head circumference.