Final answer:
Preterm labor is defined as labor that begins before the end of the 37th week of pregnancy. Oxytocin is the hormone responsible for contractions during labor, and the cervix must thin and dilate before the baby and placenta can be expelled.
Step-by-step explanation:
Preterm labor is labor that occurs before the end of week 37. The correct answer to the question is option a. 37. It is important to monitor physiological changes as a pregnancy progresses into its final weeks due to hormones that trigger labor. Before the baby and the placenta are expelled, which is known as the expulsion stage of labor, the cervix thins and dilates.
This process is led by the hormone oxytocin, which is primarily responsible for the contractions during labor. Understanding these stages is crucial, as preterm premature rupture of membranes (PPROM) can lead to labor before 38 weeks of gestation, which requires medical attention to avoid complications.