One of the dramatic recent changes in family life in Western nations has been the rise in non-marital childbearing. Much of this increase is attributable to the growth in cohabitation. But in some countries, notably the UK (and the USA) this is much less the case with significant proportions of children being born to parents who are not living together. This study uses data from the Millennium Cohort Study, a British birth cohort established in 2001, to examine whether the closeness of the tie between parents, as assessed by their partnership status at birth, is related to smoking during pregnancy, breastfeeding and maternal depression. Four sets of parents are distinguished representing a hierarchy of bonding or connectedness: married and cohabiting parents, and two groups of solo mothers, those closely involved with the father at the time of the birth and those not in a relationship. Smoking in pregnancy, breastfeeding and maternal depression tests for trend, adjusted for socio-demographic factors, showed that there was a statistically increased risk of adverse health and health behaviours by degree of parental connectedness. There were also consistent and statistically significant differences between married and non-married mothers. Particularly noteworthy was the finding that cohabiting mothers have greater risk of adverse outcomes than married women. Among the non-married set, there were also differences in risk of adverse outcomes. For smoking in pregnancy, the key difference for continuing to smoke throughout the pregnancy lay between mothers involved with partners and those lacking an intimate relationship. For breastfeeding, stronger parental bonds were associated with initiation of breastfeeding, with a clear difference between cohabiting mothers compared to solo mothers. There was also an increased risk of maternal depression with looser parental bonding, and among non-married groups this increased risk was most noticeable among cohabiting mothers when compared with solo mothers.