The aim of the study was to measure pelvic floor muscle function in continent and incontinent nulliparous pregnant women. The study group consisted of 103 nulliparous pregnant women at 20 weeks of pregnancy. Women reporting urinary incontinence once per week or more during the previous month were classified as incontinent. Function was measured by vaginal squeeze pressure (muscle strength) and increment in thickness of the superficial pelvic floor muscles (urogenital diaphragm) assessed by perineal ultrasound. Seventy-one women were classified as continent and 32 women as incontinent. Continent women had statistically significantly higher maximal vaginal squeeze pressure and increment in muscle thickness when compared with incontinent women. There was a strong correlation between measurements of vaginal squeeze pressure and perineal ultrasound measurements of increment in muscle thickness. This study demonstrates statistically significant differences in pelvic floor muscle function measured by strength and thickness in continent compared with incontinent nulliparous pregnant women.