Embryology
Lec: 7 Prof Dr. Al-Hubaity
Fetal Period:
It extends from the beginning of the 3
rd
month to the 10
th
lunar month,
that is to say it extend from the 9
th
week to the end of gestation (Up to
Delivery).
Fetal period is the period of maturation & it is considered as growth
period where the embryo grow both in length & weight.
By the end of 3
rd
month (After 12 weeks) the fetus show the following
features:
1. The head: is the biggest part of the body, constitutes about 1/2 of
the Crown-Rump of the fetus.
2. The face: becomes more human in appearance (very clear) because
the eyes now becomes ventrally and near the midline (first it is
located laterally), while the ear becomes laterally located (first it is
located dorsally) and the nose & mouth become clear.
3. The limbs: are completely formed and having their normal
position (they were straight and short).
4. The external genitalia: now we can differentiate between male
and female.
5. The nervous system: shows some activity and reflexes.
4
th
-5
th
Month:
After the 3
rd
month, we can assess the age of fetus by knowing its length
as Crown-Rump length, or Crown-Heel length (which is the length from
vertex of skull to the sole).
By the end of the 5
th
month, the fetus show the Following features:
1-The head:
is about one-third (1/3) of the Crown-Heel length. At this
period, the Crown-Rump length is 15cm, while the Crown-Heel length is
23cm (The head is about 8cm).
The length of the newborn baby s about 48-50cm, therefore the length of
the fetus by the end of the 5
th
month if about the half of its length at
delivery.
At birth (or just before it) the Head is12.5-13cm which is about 1/4 of the
total Crown-Heel length.
2-The Weight:
is about 500-600gm. The average weight of the new
born is 3-3.5Kg (average 3.25 Kg). So it is about 1/7 of its weight at
delivery.
The more gaining in weight takes place during the last 1/4 of pregnancy,
i.e. the last 2.5 months before delivery. At this period, about 50% of the
weight is added. At the end of the 7
th
month (29-30) weeks, its weight is
about 1650-1750 gm & the majority of the other 50% weight is added
during the last 10 weeks.
On the other hand, the weight of the placenta at the end of pregnancy (at
delivery) is about 500-600 gm.
Factors Affecting Weight Gaining of the Fetus:
1- Genetic constitution (male or female): Male newborn weight is
about 3350 gm. Female newborn weight is about 3100 gm.
2- Placenta efficiency: in cases of placenta insufficiency or ischemia,
the weight of the new born will be less than 3 kg.
3- Some habits of the pregnant mother: like smoking.
4- Some diseases affecting the pregnant mother.
5- Nutrition State.
Light for Date:
The weight of the new born is less than normal at the time of delivery,
although has completed full term gestation.
Big for Date:
The weight of the newborn is more than normal, usually associated with
gestational diabetes. After the delivery some years later, the mother is
discovered to be diabetic
Premature:
A newborn baby born before completing the full gestation, is called
premature. When the baby is born after 28 weeks, it will probably be able
to survive.