Fetal development: Nursing

Prenatal development usually lasts 38 to 40 weeks, and it can be subdivided into three main stages. The first stage, called the preembryonic stage, starts with conception and ends with implantation, lasting for 2 weeks. The second stage, or the embryonic stage, lasts from the third week through the eighth week, and it’s when the embryo develops until it becomes a fetus. The last and the longest stage of prenatal development is called the fetal stage, and it lasts from the ninth week until birth. Now, these stages are calculated based on fertilization age, meaning how much time has passed since the last ovulation, when the fertilization of the oocyte by the sperm took place. On the other hand, gestational age is measured from the first day of the mother’s last menstrual period, so it’s usually 2 weeks longer than fertilization age.

Let’s start with the preembryonic stage, which covers the first two weeks in development. This stage starts with conception, more specifically, with fertilization; this is when a sperm meets the egg in the fallopian tube and they fuse, forming the zygote. The zygote then makes its way through the fallopian tube in order to reach the uterus; and at the same time, it undergoes mitosis, meaning it divides over and over again into smaller cells called blastomeres. When there are 32 blastomeres, the fetus is called a morula. Next, a fluid cavity appears inside the morula to form the blastocyst. Eventually, the blastocyst reaches the uterus and implants in the inner lining of the uterus, called the endometrium.

Next comes the embryonic stage, which lasts from the beginning of the third week of gestation to the eighth week. The first major change in this phase is that the embryo develops its three primitive germ layers; the ectoderm, the mesoderm, and the endoderm, so it starts looking like 3 pancakes on top of one another. And throughout the embryonic phase, these three layers give rise to all the organs and tissues of the embryo. Cells from the ectoderm develop into the skin and nervous system, including the brain, spinal cord and peripheral nerves; but also the bones that form the skull. Most other bones and skeletal muscles come from the mesoderm, which also give rise to the heart, circulatory system, the kidney, gonad and adrenal glands. Finally, the gastrointestinal and respiratory system comes from the endoderm, but also endocrine organs like the thyroid and pancreas. During this time, as the embryo grows, it folds in on itself, both on its longitudinal axis, and its transverse axis. So by the end of week 8, the embryo is similar to a kidney bean in terms of both size and shape.

Overall, this is a very exciting, but also a very vulnerable stage, because if a teratogen like a medication or illicit drug enters the body, that can cause the organs to develop abnormally, leading to congenital malformations.

Okay, now, let’s switch gears and look at the fetal stage, starting with weeks 9 through 12. During these weeks, the fetal head makes up half of the fetal length, while the lower limbs are still shorter than the upper ones. At this point, the fetus has fused eyelids, and the nasal septum and palate are fully developed, while the permanent teeth buds start to grow under the primary teeth. The central nervous system is fully formed but the fetal brain is smooth, without sulci and gyri. During these weeks, the production of blood cells occurs within the liver, spleen and lymphatic tissue. During this stage, the heart is fully developed and the heartbeat can be detected with a fetal doppler ultrasound.

The development of the digestive tract is completed; the intestinal loops are within the abdominal cavity; and the liver starts bile production. Kidneys move up from the pelvis, into the abdominal cavity and start urine production and excretion. Finally, at 12 weeks, the genitals are well differentiated as male or female; spontaneous fetal movements occur; and the crown-rump length, meaning the measurement of the fetus from the top of the head to the bottom of the buttocks, is usually around 55 millimeters.

Next, the time between the 13th and 16th week is a period of rapid growth. The fetal skin is thin, with visible blood vessels; and lanugo, which is a type of soft, fine fetal hair, begins to develop. Also, during this period, the fetus starts to swallow amniotic fluid and produce meconium, which is the fetus’s first stool that’s black or green in color.