Abstract We explore breastfeeding practices among low-income families in the late 18th to early 19th-century town of Rauma in Southwestern Finland. The breastfeeding practices in the area of the current nation of Finland (at the time belonging to first Sweden and then Russia) had been under debate. While in certain regions artificial infant feeding was common and was linked to high infant mortality, breastfeeding was also known to be practiced in certain regions of Finland. To explore this, we analyzed the δ15N and δ13C values in collagen of horizontally cut dentin segments of permanent first molars (M1; n = 7) collected from 19th century human skeletal remains from the deconsecrated Holy Trinity churchyard excavated in 2016. The resulting isotopic profiles were similarly patterned across the seven individuals. The emerging pattern revealed a period of exclusive breastfeeding during approximately the first six months of life, followed by weaning until the latter half of the second year. We further investigated diet during mid-childhood by comparing the stable isotope ratios in collagen of the M1 and premolar roots. This comparison suggested that the mid-childhood diets may have contained more plant-based foods than those consumed earlier in childhood.