Week 20 (CRL= 160 mm)
At this period, the heart sounds can now be auscultated using a regular fetoscpe. Brown fats, the special heat-producing fat, are deposited at this time. Presence of brown fats enables the newborn to maintain a stable temperature after birth. The fetus weighs about 460 grams by the twentieth week.
- NERVOUS and SENSORY: The myelination of the nerves initiates. This process continues throughout the first year of life.
- CARDIO and RESPI: The heart beat can now be detected using a regular fetoscope.
- DIGESTIVE: The peristalsis is already well developed.
- GENITOURINARY: 40% of the nephrons are now mature and functioning. The testes is contained in the abdominal area, however, it is starting to descend towards the scrotum. Regarding the ovary, at this point it reaches it peak amount of primordial follicles which is 5 to 7 million and then gradually declines.
- MUSCULOSKELETAL: Fetal movements are already felt by the mother and may be palpable by an experienced examiner.
- INTEGUMENTARY: The thin skin is covered with vernix caseosa. Brown fat production is already completed and the nipple development initiates.
Week 24 (CRL= 230 mm)
The fetus continues to grow and gain weight at this period. The subcutaneous fat deposition is still minimal thus, the fetus still appears thin. An important event during the twenty-fourth week is the production of surfactant by the lungs. Surfactant is a surface-active lipid substance that facilitates lung expansion. It minimizes surface tension in the alveoli making it easier for the baby to breathe after birth. A fetus born at this gestation has less chance of survival because of the inadequate gas exchange due to the immature body systems. Weight is approximately 820 grams.
- NERVOUS and SENSORY: The spinal cord ends at the level of the first sacral vertebra due to the rapid growth of the vertebral canal.
- CARDIO and RESPI: Capillary network surrounds the developed primitive thin-walled alveoli or air sacs. Surfactant production in the lungs is initiated making respiration possible. However, most fetuses born at this time die because of the immaturity of the body systems.
- GENITOURINARY: The testes are now descending toward the inguinal rings.
- MUSCULOSKELETAL: Fetal active movements are noted. These movements are progressively noticeable to the mother and examiner.
- INTEGUMENTARY: Lean body appearance is notable. The skin appears red and wrinkled because the capillaries are close to its fragile surface. Fingerprints and footprints are already developed. Fingernails are now present and the eyebrows and eye lashes are also present.