In low fertility settings, religious people tend to have larger families than non-religious people. One way religious individuals may achieve larger relative family sizes is through support from their families. In this paper, we investigate the relationships between religiosity, kin contact, allomaternal investment from relatives, and fertility in two high income low fertility settings: the United Kingdom and the United States. Data for this pre-registered research come from an online survey of 609 women living in the US and 919 women living in the UK, recruited through Prolific, who answered questions about their religious practices, childbirth histories, social networks, and allomaternal networks. We find that, compared with less religious peers, more religious women: 1) have more geographically diffuse kin networks (particularly in the UK) but have social networks that are equally kin-dense; 2) receive more allomaternal support from kin beyond their partner, particularly help with household tasks, though the countries differ in the exhibited relationship between religiosity and partner support; and 3) have higher fertility in both countries. We do not find strong evidence for a mediating role of allomaternal support on the relationship between religiosity and fertility. Our study highlights important variation in the relationship between religion and fertility across two high income low fertility countries and raises new questions about the role that religion plays in allomaternal support networks in these settings.