Week 28 (CRL= 270 mm)
More likely, the fetus will survive if born at this period because of the maturation of the lungs, pulmonary capillaries and central nervous system. Blood formation shifts from the spleen to the bone marrow and more subcutaneous fat is deposited under the skin. At this time, the fetus assumes a head-down position (for normal birth). Weight is approximately, 1300 grams.
- NERVOUS: Major sulci and gyri are present.
- SENSORY: After the 26th week, the eyelids are no longer fused. The fetus now responds to bitter substances in the tongue.
- CARDIO and RESPI: Erythrocyte (RBC) formation is shifted completely to the bone marrow from the spleen. Adequate alveoli, surfactant and capillary network allow respiratory function to be possible although respiratory distress syndrome is common. Many infants born at this time survive with an intensive care.
- GENITOURINARY: The testes have already descended into the inguinal canal into the scrotum by the end of the 26th week.
- INTEGUMENTARY: The skin is slightly wrinkled but is smoothing out as subcutaneous fat is deposited under it.
Week 32 (CRL= 300 mm)
The fetus weighs about 2100 grams at this period and the chances of survival for fetus born at this time are good.
- NERVOUS and SENSORY: The maturation of the parasympathetic nervous systems nears that of the sympathetic nervous system. This results to the greater fetal heart rate variability on electronic fetal monitor tracing.
- CARDIO and RESPI: The surfactant production nears the mature levels. However, respiratory distress is still possible if the fetus is born at 32 weeks age of gestation.
- INTEGUMENTARY: The skin is now smooth due to more subcutaneous fat deposits. The body contour appears rounded due to the additional fat deposition under the skin. The skin is pigmented according to the race. Larger vessels are visible over the abdomen and beneath the skin. Toenails are present and the fingernails extend to the fingertips. The lanugo is now disappearing.
Week 38 (CRL= 360 mm)
The rate of growth slows down as full term approaches. At this point, the fetus is mainly gaining weight. Because of the mature pulmonary system efficient and unlabored breathing after birth is enabled.
- NERVOUS and SENSORY: Sulci and gyri are already developed. The visual acuity after birth is 20/600.
- CARDIO and RESPI: About one eighth (1/8) to one sixth (1/6) the number of alveoli of an adult is present in a newborn infant. The ability to exchange gases is now well-developed.
- GENITOURINARY: The testes are both palpable in scrotum at birth. In females, the ovaries contain 1 million follicles at this point. After birth, no new follicles are formed and their numbers continue to decline after birth.
- INTEGUMENTARY: The fetus appears plump and the skin is smooth. Vernix caseosa is present in the major creases of the body. The lanugo (fine downy hair) is only found in the shoulders and upper back. Ear cartilages are firm at this point and the fingernails are extending beyond the fingertips.
image courtesy of nyp.org, trjohn.blogspot.com, embryology.med.unsw.edu.au, abort73.com