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Objectives: This study was performed to determine whether multiparous pregnant women are prone to influenza. Methods: A questionnaire survey was conducted at 19 centres located throughout Japan, targeting all 6694 postpartum women within 7 days after birth before leaving the hospital. All women gave birth during the study period between March 1, 2015, and July 31, 2015. Data regarding vaccination and influenza infection in or after October 2014, age, previous experience of childbirth, and number and ages of cohabitants were collected. Results: Seventy-eight percent (n = 5,197) of women given questionnaires responded. Of these, 2,661 (51%) and 364 (7.0%) women reported having been vaccinated and having contracted influenza, respectively. Multiparous women had a higher risk of influenza regardless of vaccination status (8.9% [121/1,362] vs. 5.7% [74/1,299], relative risk [95% confidence interval], 1.80 [1.36 to 2.38] for vaccinated and 9.3% [112/1,198] vs. 4.3% [57/1,328], 2.18 [1.60 to 2.97] for unvaccinated women) compared to primiparous women. The risk of influenza increased with increasing number of cohabitants: 4.8% (100/2,089), 7.5%, (121/1,618), 9.0%, (71/785), and 10.4% (58/557) for women with 1, 2, 3, and ≥ 4 cohabitants, respectively. Conclusions: Family size is a risk factor for influenza infection in pregnancy.