The fetal period

During the embryonic period, the development of the brain and the corresponding enlargement of the head were predominant. The fetal period is notable for the rapid growth of the other parts of the body. Thereby, the body proportions change. At the beginning of the fetal period, the head occupies approximately half of the CRL. At month 5, it is a third of the CHL. At birth the head is only a quarter of the CHL.
In a fetus of 9 weeks, the physiological umbilical hernia has disappeared. The intestinal loops have returned into the abdominal cavity. Towards the end of month 3, the external genital organs have developed such that the sex of the baby can be diagnosed through ultrasound. By month 4, hairs appear in the region of the eyebrows and the lips, and as fine lanugo hairs on the body surface. During month 5, usually the mother can clearly feel the fetal movements (quickening). Around the middle of pregnancy, the skin on the fetus is wrinkled because the subcutaneous adipose tissue is still missing. Subcutaneous fat begins to accumulate in the last months of pregnancy, causing wrinkles to disappear. Shortly before birth, the sebaceous glands excrete a whitish, greasy substance called the vernix caseosa. From week 26 onward, the lungs have matured to such an extent that the child can survive outside the uterus (preterm delivery). An abortion is when pregnancy is terminated before viability, i.e. before the fetus is sufficiently developed to survive outside the uterus.
In obstetrics, the date of birth is usually calculated from the first day of the last menstrual period (LMP). The time interval between this day and ovulation is around 14 days. Since this interval is variable, in embryology the ovulation date is used as the key reference point.
It is possible nowadays to follow the growth of the baby and, sometimes, to identify malformations through the use of ultrasound, an important element in prenatal diagnosis.