embryo-3rd wk to birth

DEVELOPMENT OF FETUS

• Fetal period, 9th week (3rd month) to birth

• Maturation of tissues and organs and rapid growth of the

body

• Length of fetus, crown-crump length (CRL) or crown-heel

length (CHL), expressed in centimeters, correlated with the

age of fetus in weeks.

A. CROWN-CRUMP LENGTH (CRL)

• sitting height

• from vertex of skull to the heel

• up to 36 cm (for a full term baby) at 37-

38 week

B. CROWN- HEEL LENGTH (CHL)

• standing height

• from vertex of skull to heel

• up to 51 cm

Beginning of the third month: the head constitutes

approximately half of the CRL.

• Beginning of the fifth month: the size of the head is about

one third of the CHL, and at birth, it is approximately one

quarter of the CHL.

• Hence, over time, growth of the body accelerates but

that of the head slows down

• Growth in length is particularly striking during: 3rd, 4th, and

5th months.

• An increase in weight is most striking during the last 2

months of gestation.

• In general, the length of pregnancy is considered to be 280

days, or 40 weeks after the onset of the last normal

menstrual period (LNMP) or, more accurately, 266 days or

38 weeks after fertilization.

• Age is calculated from the time of fertilization and is

expressed in weeks or calendar months.

monthly changes

3

rd month to birth, the progress or the development of the

fetus is accounted month by month

• 3

rd month – primary ossification centers, external genitalia -

the testicles inside the scrotum is visible via the ultrasound

in the fetus.

o Fetus during 3rd month has the look of a human

being (with the face, the eyes etc) but the skin is

very thin that the blood vessels inside are

noticeable from outside.

o the face becomes more human looking. The limbs

reach their relative length. external genitalia

develop to such a degree that the sex of the fetus

can be determined by external examination

(ultrasound). At the sixth week, intestinal loops

cause a large swelling (herniation) in the umbilical

cord.

4

th, 5th month – lanugo hair, fetal movement felt by mother.

o If a mother goes to the doctor for a prenatal check-

up, and they’re not sure when the last menstrual

period was, then the month of pregnancy can be

determined by whether or not they can feel the fetal

movement.

o The fetus is covered with fine hair, called lanugo

hair; eyebrows and head hair are also visible.

• 6

th month-respiratory system and central nervous system

not yet well established, have differentiated fully, great

difficulty in surviving.

o The skin of the fetus is reddish and has a wrinkled

appearance (because of the lack of underlying

connective tissue).

7

th month – developmental events occur, 90% chance of

survival, the respiratory system, the sucking and the

swallowing mechanisms are already developed which is why

the survival is better than the 6th month.

• 8

th month- vermix caseosa, whitish fatty substance. When a

baby is delivered with whitish material covering the body, the

baby has crossed the 8 month mark.

9

th month- skull largest circumference of all parts of the

body. If the baby is born in breech position (feet first), the

circumference of the head should be known to confirm

whether or not the mother will be able to deliver the head.

• At 18th week, the fetus has subcutaneous tissue and the

attachment to placenta

• At the time of birth, the full term baby is 38 weeks after

fertilization of 266 days (38x7) or 40 weeks. This is computed

from the last normal menstrual period.

• Ovulation happens for 24hours but the sperm travels for 6

days. Not all eggs are fertilized right after ovulation within 24

hours.

• Window period is of about 6 days, and implantation is for 6-

7 days, so there’s an allowance of 10-14 days.

• During the last 2 months, the fetus obtains well-rounded

contours as the result of deposition of subcutaneous fat.

• At the time of birth, the weight of a normal fetus is 3,000 to 3,400 g, its CRL is about 36 cm, and its CHL is about 50 cm.

Sexual characteristics are pronounced, and the testes

should be in the scrotum

TIME OF BIRTH

Age of gestation:

• 266 days or 38 weeks after fertilization or 40

weeks from the first day of the LNMP. In women

with regular 28-day menstrual periods, the

method is fairly accurate, but when cycles are

irregular, substantial miscalculations may be

made.

• An additional complication occurs when the

woman has some bleeding about 14 days after

fertilization as a result of erosive activity by the

implanting blastocyst. Hence, the day of delivery

is not always easy to determine. Most fetuses are

born within 10 to 14 days of the calculated

delivery date.

• If they are born much earlier - Premature

• If born later – Postmature

• Ultrasound- A valuable tool for assisting in this

determination, which can provide an accurate (1 to 2

days) measurement of CRL during the 7th to 14th

weeks.

• Measurements commonly used in the 16th to 30th

weeks:

o Ultrasound (determines the age of gestation),

CRL, BPD (biparietal diameter), head,

abdominal circumference, femur length

• Average size of newborn: 2,500-4,000g: 51 cm length

• An accurate determination of fetal size and age is

important for managing pregnancy, especially if the

mother has a small pelvis or if the baby has a birth

defect

• Low Birth Weight (LBW): weight <2,500g regardless

of gestational age

Intrauterine Growth Restriction (IUGR): infants who do

not attain optimal intrauterine growth; pathologically

small, at risk for poor outcomes; gestational age taken

into account.

• Small for Gestational Age (SGA): birth weight below

10th percentile for gestational age; healthy but smaller

in size. no poor outcomes. Gestational period is

taken into account also.

• Intrauterine Growth Restriction (IUGR): infants who do

not attain optimal intrauterine growth; pathologically

small, at risk for poor outcomes; gestational age taken

into account.

• Small for Gestational Age (SGA): birth weight below

10th percentile for gestational age; healthy but smaller

in size. no poor outcomes. Gestational period is

taken into account also.

PLACENTA

Facilitates nutrient and gas exchange between maternal and

fetal compartments

• As the fetus begins the 9th week of development, its demand

for nutritional and other factors increase, causing the major

changes in placenta.

• The deposition of fetal membranes is also altered as

production of amniotic increases.

• Maternal blood is delivered to the placenta by spiral arteries

in the uterus.

• Erosion of these maternal vessels to release blood into

intervillous spaces is accomplished by endovascular

invasion by cytotrophoblast cells.

• Fetal component of placental is derived from trophoblast and

extraembryonic mesoderm, chorionic plate

• Maternal component is derived from uterine endometrium

• Fetal side covered by chorionic plate. Fetal side also has

blood vessels covered by the amnion connecting the fetus

and the placenta with the umbilical cord.

• Chorion frondosum, chorionic plate (lines the trophoblast)

• Decidua basalis, decidual plate

• Junctional zone, decidual septae, compartments or

cotyledons

III. TIME OF BIRTH

IV. PLACENTA