The fertilized egg travels through the fallopian tube for four days before reaching the uterine cavity. It then remains freely within the uterine cavity for approximately three days, waiting for the endometrium to prepare itself. Once ready, it finds a suitable location to implant itself—a process known as implantation. The fertilized egg undergoes multiple divisions, forming a cell cluster that gradually grows while beginning to differentiate. Part of it develops into the fetus, while another part becomes accessory organs that supply nutrients to and protect the fetus. This marks the first week of life.
During the second week, the tiny life grows remarkably fast. The spine forms, and rudimentary structures emerge for the brain tissue, spinal cord, nervous system, and eyes. The other end of the spine features a small tail. Blood vessels begin to develop, though the heart itself is not yet formed. However, a heartbeat can be detected at the site where the heart will eventually develop.
By the fourth week, the heart appears. One week later, limb buds emerge, followed by the eyes and ears. The lungs and liver also begin to take shape. The human brain increases in weight at a notably faster rate than in other animals.
By the seventh week, fingers and toes begin to form on the hands and feet. The eyes become clearly visible. The embryo straightens and moves in the amniotic fluid, taking on an appearance resembling a small child. The tail disappears. By the end of the eighth week, the embryo measures nearly 3 centimeters, and all organs have taken on their basic forms. The following months are dedicated to gradual refinement.
Entering the third month (weeks 8-11), the developing life is now termed a fetus. By the beginning of week 8, the fetal head accounts for half the total body length. Growth accelerates thereafter, with body weight doubling by the end of week 12. Internal organ systems begin functioning, enabling the fetus to swallow amniotic fluid, which is then excreted as urine. At week 9, the external genitalia of male and female fetuses appear largely similar. By the end of week 12, the distinct external genitalia of mature fetuses begin to emerge.
By the fourth month (12-15 weeks), the fetus reaches 16 cm in length and weighs approximately 120 grams, growing rapidly. The disparity between head and body proportions diminishes, legs lengthen relatively, and bones ossify rapidly. Under the influence of the liver, stomach, and intestines, green meconium forms, which will be expelled after birth. Lanugo hair appears on the skin. The heart rate is twice that of an adult.
Fifth Month (16-19 weeks): The mother may feel the fetus kicking inside her abdomen, signaling its presence—this is fetal movement. Another sign is the fetal heartbeat, audible over the abdomen, typically ranging from 120 to 160 beats per minute. The fetus has developed hearing and can perceive sounds, making this an ideal time to begin prenatal education. At this stage, the fetus measures about 25 cm in length and weighs 500 grams.
By the sixth month (20-23 weeks), the fetus measures about 30 cm in length and weighs 660 grams. Both arms are bent across the chest, and both knees are drawn up toward the abdomen. At this stage, birth would typically result in survival for only a few hours, as the respiratory system remains underdeveloped. In the seventh month (24-27 weeks), the fetus measures approximately 35 cm in length and weighs 1000 grams, resembling a little old man.Though capable of shallow breathing and crying, survival remains highly unlikely.
Eighth month (28-31 weeks): The fetus measures about 40 cm in length and weighs approximately 1700 grams. It moves freely within the uterus with coordinated movements and a largely fixed position, typically head-down.The nervous system further matures, and the lungs and other internal organs are largely developed. Premature infants born at this stage can survive with intensive care in an incubator. Ninth Month (32-35 weeks): The fetus measures about 45 cm in length and can gain 1000 grams within four weeks, with development largely complete. Premature infants born at this stage have a survival rate exceeding 90% with careful care.
Tenth Month (36–40 weeks): Fetal development is complete, with a length of about 50 cm and a weight of 3000 g. The skin appears white with a slight pinkish hue, covered by a layer of white adipose tissue. The chest is well-developed with prominent breasts. The fetus can hiccup and suck its thumb. In males, the testes are usually descended into the scrotum.
Although labor may last from several hours to a day or longer, the fetus endures significant challenges. A fully developed fetus can tolerate oxygen deprivation, compression, and other stresses to achieve successful delivery. Vaginal delivery typically takes longer than cesarean section but enhances the newborn's ability to adapt to the external environment.