What occurs in the body to signify the beginning of labor?

The beginning of labor is marked by the contraction of the uterus. These contractions, known as Braxton Hicks contractions initially, progress to the regular and rhythmic contractions that characterize true labor. These contractions help to dilate the cervix and facilitate the birth process. Hormones such as oxytocin play a significant role in stimulating these uterine contractions, which help to push the baby down the birth canal.

The other options listed do not play a role in the onset of labor. The expansion of the foramen ovale and the closure of the ductus arteriosus are events that relate to the transition of the circulatory system after birth, while the development of the neural tube pertains to embryonic development, occurring well before the labor process begins. Each of these processes is essential in its own context but does not indicate the start of labor.