Lecture 7 - Fetal Period and Twins - Prof. Raouf Fadel - 29 April 2024.pdf

THIRD MONTH TO BIRTH “FETAL PERIOD” Prof. Raouf Fadel 2024 The fetal period is characterized by the rapid growth of the body and maturation of organs. The weight of the fetus increases from 30 gm at the beginning of this period to reach about 3 kg at t...

THIRD MONTH TO BIRTH “FETAL PERIOD” Prof. Raouf Fadel 2024 The fetal period is characterized by the rapid growth of the body and maturation of organs. The weight of the fetus increases from 30 gm at the beginning of this period to reach about 3 kg at the full-term. The length (Crown-heel) of the fetus is about 50 cm at birth The sex of the fetus could be determined from the end of the third month (12 weeks) due to the appearance of external genitalia (ultrasound) The movement of the fetus could be felt by the mother from the end of the fourth month (16th week) The fetal hear t beat is heard through the stethoscope from the beginning of the fifth month (after the 16th week) The fetus could live if he was born after 24 weeks (6 months) of development. The length of pregnancy for a full-term fetus is considered to be 280 days or 40 weeks after the onset of the last menstrual period. The age of a full-term fetus is considered to be 266 days or 38 weeks after fertilization (2 weeks less than length of pregnancy). Methods of Estimation of The Embryonic and Fetal Ages Embryonic and fetal ages could be estimated by using the ultrasound. The most common measurements are the crown rump length (sitting height), crown heel length (standing height), head circumference and biparietal diameter. CRL The size of the head constitutes: a- 1/2 of the fetal length at the 3rd month b- 1/3 of the fetal length at the 5th month c- 1/4 of the fetal length at birth. 20-day albino rat fetuses Congenital Anomalies Many factors may interact with the differentiating and growing of the embryo. 1. These factors could be environmental (e.g. drugs and viruses), genetic (chromosome abnormalities) or both. 2. Agents which are introduced during pregnancy and produce congenital anomalies are known as Ter atogens (like alcohol, drugs, virus….). 3. The most sensitive period for teratogenesis is the embr yonic per iod where various structures are first formed and differentiated. Dizygotic Twins (2/3 ) It results from fertilization of two ova by different sperms. 1. They are non-identical twins (unlike twins) that may have same or different sexes and have different blood groups. 2. They always have two amnions, two chorions and two placentae (figure A) 3. The chorions and placentae may fuse (Figure B). Monozygotic Twins (1/3) It results from a single fertilized ovum. The twins are identical with same sex ,blood group and physically. A- If separation at two cell stage: separate chorionic sacs, amniotic cavities and placentae. Monozygotic Twins (1/3) B-If separation at blastocyst stage: the inner cell mass will divide into two complete separate embryonic groups. The two embryos will have common chorionic sac and common placenta and two separate amniotic cavities (the most common). Monozygotic Twins (1/3) C- If separation of inner cell mass is incomplete : the embryos will share the same amniotic cavity (rare)