Fetal hearing development starts around the second trimester. By approximately 18-20 weeks, the fetus begins to detect sounds. Initially, these sounds are muffled due to the amniotic fluid and the mother's tissues. The fetus can hear low-pitched sounds better than high-pitched ones. Common sounds experienced by the fetus include the mother's heartbeat, digestive system, voice, and external noises like music or conversation. This auditory exposure is crucial for auditory development and can influence postnatal recognition of familiar sounds. Research suggests that fetuses may even respond to these sounds by moving or increasing their heart rate, indicating a level of awareness and interaction with the external auditory environment.

During the first trimester, a woman's body undergoes numerous changes to accommodate the developing embryo. Hormonal changes are profound, with increased levels of hormones like hCG, progesterone, and oestrogen. These hormonal shifts can lead to symptoms such as morning sickness, fatigue, breast tenderness, and frequent urination. The uterus begins to enlarge to provide space for the growing embryo. Additionally, there's an increase in blood volume and changes in blood pressure to support the growing fetus. These changes are crucial for creating a suitable environment for the embryo to implant and develop, marking the beginning of the journey towards childbirth.

In the third trimester, the fetus undergoes significant developments, preparing for birth. The fetus's size and weight increase considerably, with fat accumulating under the skin, providing insulation and energy reserves. The lungs mature, preparing the fetus for breathing air after birth. The brain continues its rapid development, enhancing the ability to control body temperature and breathe. Reflexes, such as sucking and swallowing, develop, essential for feeding after birth. The bones harden, yet the skull remains soft and flexible to facilitate passage through the birth canal. The fetus usually moves into a head-down position in preparation for delivery. This trimester is crucial for completing the maturation of organ systems and ensuring the fetus is ready for life outside the womb.

Amniotic fluid plays several vital roles in pregnancy. It surrounds the fetus in the amniotic sac, providing a cushion against external pressure and shock. This fluid facilitates fetal movement, crucial for musculoskeletal development. It also maintains a consistent temperature around the fetus, protecting against temperature fluctuations. In the early stages, the fluid is primarily water from the mother's body. As pregnancy progresses, the fetus contributes to the fluid by urinating into the amniotic sac. The composition of the fluid changes over time, containing nutrients, hormones, and antibodies by the third trimester. It also assists in lung development, as the fetus inhales and exhales the fluid, preparing the lungs for breathing air after birth. Monitoring amniotic fluid levels is crucial, as abnormalities can indicate potential complications.

The umbilical cord is a pivotal structure in pregnancy, acting as a conduit between the fetus and the placenta. It comprises two arteries and one vein encased in a gelatinous substance known as Wharton's jelly. The arteries carry deoxygenated blood and waste products from the fetus to the placenta, where waste is expelled, and nutrients and oxygen are absorbed. The vein then carries oxygenated blood and nutrients back to the fetus. This exchange is crucial for fetal development, as it ensures a continuous supply of oxygen and essential nutrients while simultaneously removing carbon dioxide and metabolic waste. The umbilical cord's unique structure prevents the vessels from compressing, ensuring steady blood flow throughout pregnancy.