The third trimester of pregnancy takes place during weeks 29 and 40. During this stage of pregnancy, women continue to gain weight, the fetus moves regularly, and the fetal head descends. The descent of the fetal head is important in easing maternal breathing and positioning the fetus for future delivery.

Morning sickness is the feeling of nausea and vomiting that occurs in many women during the first trimester of pregnancy. The physiological causes behind morning sickness include low blood sugar, an increased sensitivity to odors, and increased levels of estrogen and gonadotropin-releasing hormone. It is believed that morning sickness evolved in order to prevent the mother from ingesting toxins that could harm the developing fetus. This is supported by the fact that during the time morning sickness occurs, the fetus is highly sensitive to toxins.

Common symptoms and indicators of pregnancy include swelling, increased urination, and back pain. Vaginal tearing is not an indicator or symptom of pregnancy, instead it is a possible occurrence during childbirth when the child exits the vagina.

The placenta is a structure that allows for nutrient, waste, and oxygen exchange between the mother and developing fetus during pregnancy. It is connected the fetus by the umbilical cord and contains umbilical arteries and umbilical veins to facilitate exchange. The placenta is derived from the blastocyst and maternal uterine lining starting at implantation and develops throughout pregnancy. The placenta is expelled from the mother immediately after childbirth.

The trophoblast is a layer of cells that surround the blastocyst. The trophoblast becomes the outer layer of the placenta and further differentiates into syncytiotrophoblast and cytotrophoblast cells that contribute to placentation.

During fetal development, the umbilical cord is a structure that connects the fetus to the maternal placenta. The umbilical cord contains umbilical arteries and the umbilical vein, allowing oxygen, nutrient, and waste exchange between the mother and fetus. It develops from the yolk sac and allantois in the fifth week of development and is clamped or cut following childbirth.

Braxton Hicks contractions are mild and irregular uterine contractions that are believed to prepare the mother for childbirth. The contractions may begin during the first trimester, but are often only noticed during the second and third trimesters of pregnancy.

During labor, uterine contractions help dilate the cervix and descend the baby into the birth canal. As labor progresses, contractions become regular and occur more frequently. As the contractions continue to bear down on the fetus, the labor process is aided by maternal pushing.

Uterine contractions during labor help the fetus descend into the birth canal and aid cervical dilation.

The rupture of the amniotic sac during birth is commonly said to be when a woman’s “water breaks.” This typically occurs at the beginning of labor, but sometimes has to be ruptured by a medical professional. The rupture decreases cushioning of the fetus and increases pressure of the contractions, aiding the descent of the fetus into the birth canal.