BACKGROUND: Age and presence of previous delivery are inextricably linked to breastfeeding. This study aims to examine breastfeeding during the first year after childbirth by combining four factors: primiparas and multiparas, as well as ages <
35 years and ≥35 years. METHODS: Four hundred thirty-six healthy mothers and their infants (vaginally delivered singleton term infants with birth weight >
2500 g) were divided into four groups: (1) 134 pairs of primiparas aged <
35 years and their infant (P [<
35] group), (2) 72 pairs of primiparas aged ≥35 years and their infant (P [≥35] group), (3) 108 pairs of multiparas aged <
35 years and their infant (M [<
35] group), and (4) 122 pairs of multiparas aged ≥35 years and their infant (M [≥35] group), and the breastfeeding rate was examined at 0, 1, 2, 4, 6, 8, 10, and 12 months after childbirth. RESULTS: The percentage of the 436 cases with only breastfeeding was 59.6% at 0 months, 69.1% at 4 months, and 56.6% at 12 months. Comparing the four groups, the percentage with only breastfeeding in decreasing order was M [<
35] group (70.8%-85.8%, range of rates at each time point) >
M [≥35] group (57.9%-69.4%) >
P [<
35] group (47.8%-61.2%) >
P [≥35] group (36.1%-51.4%). CONCLUSIONS: There were difficulties with breastfeeding for primiparas, especially those aged ≥35 years. We propose that breastfeeding should be analyzed by combining maternal age and presence or absence of previous delivery together rather than as independent factors.