A 32-year-old woman is dilated 4 cm. The health care provider states that she has a roomy pelvis and the baby is in a right occiput anterior (ROA) position at +1 station. Her contractions are occurring every 10 minutes, lasting 30 to 40 seconds, and palpate mild in intensity. She is calm and relaxed. Given these data, which essential component of labor is unexpected and likely to slow the progress of labor at this time?