Embryology LC5 - 3rd Month to Birth: The Fetus and Placenta PDF

At the beginning of the third month, the head constitutes approximately TOPIC OUTLINE half of the CRL....

At the beginning of the third month, the head constitutes approximately TOPIC OUTLINE half of the CRL. By the beginning of the fifth month, the size of the head is about one I. DEVELOPMENT OF FETUS third of the CHL, and at birth, it is approximately one quarter of the CHL. II. MONTHLY CHANGES Hence, over time, growth of the body accelerates but that of the head III. TIME OF BIRTH slows down. IV. PLACENTA Growth in length is particularly striking during: 3rd, 4th, and 5th months V. UMBILICAL CORD and an increase in weight is most striking during the last 2 months of gestation. VI. AMNIOTIC FLUID VII. PARTURITION (BIRTH) I. DEVELOPMENT OF FETUS Fetal period, 9th week to birth Maturation of tissues and organs and rapid growth of the body. We can monitor the development of the fetus through the length. The length of the fetus (in centimeters), which is correlated with its age (in weeks or months), is usually indicated as: a) CROWN-RUMP LENGTH (CRL) - sitting height - starts from the crown to the rump (buttocks) of the fetus. b) CROWN-HEEL LENGTH (CHL) - standing height - starts from the crown to the heel of the fetus. It is about Figure 2. A 9-week fetus. Note the large head size compared with that of the 51 cm. rest of the body. The yolk sac and long vitelline duct are visible in the chorionic The length of the pregnancy is considered to be 280 days, or 40 weeks cavity. Note the umbilical cord and herniation of intestinal loops. One side of the after the onset of the last normal menstrual period (LNMP) or, more chorion has many villi (chorion frondosum], whereas the other side is almost accurately, 266 days or 38 weeks after fertilization. smooth [chorion laeve]. How to compute age of gestation: Ask the patient when is the first day of her last normal menstrual period, then add 7 to the day, minus 3 for the month and plus II. MONTHLY CHANGES 1 for the year (if in the last months). Ex. May 10, 2022 Age of gestation: February 17, 2023 - 3rd month to birth, the progress or the development of the fetus is Month (May/05)) 05 - 3 = 02 February accounted for, month by month. Day (10) 10 + 7 = 17 17 A. 3rd month: primary ossification centers (in the long bones and skull by the 12th week), external genitalia is being developed. Sex of the fetus can be Year (2022) 2022 + 1 = 2023 2023 determined by external examination (ultrasound). Fetus during the 3rd month has the look of a human being (with the face, the eyes etc) but the skin is so thin that the blood vessels inside are noticeable from outside. The face becomes more human looking (Figures 3 and 4). The eyes, initially directed laterally, move to the ventral aspect of the face, and the ears come to lie close to their definitive position (Figure 4) at the side of the head. The limbs reach their relative length in comparison with the rest of the body, although the lower limbs are still a little shorter and less well developed than the upper extremities. External genitalia develop to such a degree that the sex of the fetus can be determined by external examination (ultrasound). During the sixth week, intestinal loops cause a large swelling (herniation) in the umbilical cord, but by the 12th week, the loops have withdrawn into the abdominal cavity. At the end of the third month, reflex activity can be evoked in Figure 1. Measurements, expressed in centimeters, are correlated with the age aborted fetuses, indicating muscular activity. of the fetus in weeks or months. Be careful in giving medicines. Ex. Ranitidine, do not give during organogenesis, give other antacids such as Aluminum/ Full term fetus: 37-38 weeks, expected length 35-36 cm, weight around Magnesium hydroxide. Ranitidine causes alteration in the sex of 3,000 g (3 kg). the fetus. Age of a fetus to be viable is about 20 weeks, if less than 20 weeks, non viable. In 20 weeks, the length should be about 20 cm and the weight is about 500 g. Page 1 of 7 [EMBRYO] 1.05– 3RD MONTH TO BIRTH: THE FETUS AND PLACENTA- Dr. La Paz L. Peredo, MD Figure 3. An 11-week fetus. The umbilical cord still shows a swelling at its base, Figure 5. An 18-week fetus connected to the placenta by its umbilical cord. The caused by herniated intestinal loops. The skull of this fetus lacks the normal skin of the fetus is thin because of lack of subcutaneous fat. Note the placenta smooth contours. Fingers and toes are well developed. with its cotyledons and the amnion. C. 6th month: respiratory system and central nervous system not yet differentiated sufficiently, and coordination between the two systems is not yet well established. A fetus born early in the sixth month has great difficulty surviving. The skin of the fetus is reddish and has a wrinkled appearance because of the lack of underlying connective tissue. By 6.5 to 7 months, the fetus has a CRL of about 25 cm and weighs approximately 1,100 g. Figure 4. A 12-week fetus in utero. Note the extremely thin skin and underlying blood vessels. The face has all of the human characteristics, but the ears are still primitive. Movements begin at this time but are usually not felt by the mother. B. 4th and 5th month: the fetus lengthens rapidly (Figures 1 and 5), lanugo hair (5th month), fetal movement felt by the mother. At the end of the first half of intrauterine life, its CRL is approximately 15 cm, about half the total length of the newborn. The weight of the fetus increases little during this period and by the end of the fifth month is still