Which physical change in pregnancy indicates that the fetus is becoming engaged in the birth canal?

Lightening refers to the physical change during pregnancy when the fetus descends into the pelvis and becomes engaged in the birth canal in preparation for labor. This change typically occurs in the later weeks of pregnancy and is often described by expectant mothers as a feeling of increased ease in breathing, as the pressure on the diaphragm is relieved when the fetus moves down. Concurrently, this descent can lead to increased pressure on the bladder, resulting in more frequent urination.

Other options, while related to various stages and signs of pregnancy, do not indicate engagement in the birth canal. Quickening describes the first noticeable movements of the fetus, which occurs earlier in the pregnancy and does not relate to the position of the fetus in the birth canal. Ballottement is a method of palpation used during a physical exam to assess fetal movement but does not specifically indicate engagement. Effacement involves the thinning and shortening of the cervix as the body prepares for delivery but, like ballottement, is part of the labor process rather than indicating the position of the fetus in the birth canal. Therefore, lightening is specifically the change that signals engagement, marking a significant milestone in the preparation for childbirth.