This document summarizes the key stages of human development from fertilization through pregnancy. It describes the processes of cleavage, morphogenesis, differentiation and growth that occur during embryogenesis. The stages of development from morula to blastocyst to embryonic disk are outlined. During the trimesters of pregnancy, the placenta forms and the embryo embeds in the uterus while organs and limbs develop. Fetal circulation and extraembryonic membranes such as the chorion, amnion and yolk sac are discussed. The document concludes with a description of parturition and birth.

1. Submitted by-
Susmita Purkayastha
Roll No.- 25
Paper-LS 301: Molecular
Endocrinology

2.  An interaction between the male
and female gametes to form a
zygote.
 Normally occurs in the ampulla
of uterine tube within 24 hours
after ovulation

3. It refers to all the events of sexual
reproduction taking place after
fertilization , that is, after the formation
of zygote.

4. Processes of development.
 Cleavage - Cell division without growth.
 Morphogenesis - Shaping of embryo.
 Differentiation - Cells take on specific
structure and function.
 Growth - Increase in size of cells.

5. Stages of development
 Morula - Solid mass of cells resulting
from cleavage.
 Blastocyst - Ball of cells formed from
morula.
 Embryonic disk - Inner mass of cells of
blastocyst.

6.  About 6-7 days after fertilization,
the embryo embeds itself into the
wall of the uterus.
 Trophoblast forms placenta
 “Inner cell mass” forms embryo
 Progesterone hormone

7. Gastrula - Embryo composed of three
tissues:-
Ectoderm, mesoderm, endoderm.

9. Extraembryonic Membranes
 Membranes that extend out beyond the
embryo.
 Chorion- surrounds the embryo & protects it.
 Amnion - Provides fluid environment for
developing embryo and foetus.
 Yolk sac – functions as the site of early blood
cell formation.
 Allantois - Furnishing blood vessels to the
placenta.

10.  Provides large area for exchange of O2,
CO2, nutrients, metabolic wastes
between fetal and maternal blood.

11. Fetal Circulation
 The umbilical cord stretches between the
placenta and the foetus and contains the
umbilical arteries and veins.
 Exchange of gases and nutrients between
maternal and fetal blood takes place in the
umbilical arteries.
 Umbilical vein carries blood and oxygen away
from the placenta to the foetus.

12.  Pregnancy is divided into three
trimesters:
• First trimester – conception to 12
weeks
• Second trimester – 12 to 24 weeks
• Third trimester – 24 to 40 weeks.

13. WEEK CHANGES IN MOTHER DEVELOPMENT OF BABY
1 week Ovulation Occurs Fertilization occurs, cell division
begins and continues.
2 weeks Symptoms of early
pregnancy (nausea, breast
swelling
and tenderness, fatigue);
blood pregnancy tests may
show positive
Implantation occurs; amnion and
yolk sac appear; embryo has tissue;
placenta begins to form
3 weeks First period missed; urine
pregnancy test may show
positive; early pregnancy
symptoms continue
Nervous system begins to develop;
blood vessels are present and
placenta is well formed
4 weeks Limb buds form; heart is beating;
nervous system further develops;
embryo has tail; other systems are
forming

14. WEEK CHANGES IN MOTHER DEVELOPMENT OF BABY
5 weeks Uterus is the size of a
hen's egg; mother may
need to urinate
frequently
Embryo is curved, head is large,
limb buds are showing division,
nose, ears and eyes are noticeable
6 weeks Uterus is the size of an
orange
Fingers and toes are present and
skeleton is cartilaginous
8 weeks Uterus can be felt above
the pubic bone
Foetus begins to look human;
limbs are developing and major
organs forming; facial features are
becoming refined
12
weeks
Uterus is the size of a
grapefruit
Head grows faster than the rest of
the body; facial features are
apparent, but there is no layer of
fat yet and the skin is translucent;
gender can be distinguished via
ultrasound; fingernails appear

16. 8 weeks
10 weeks

17. WEEK CHANGES IN MOTHER DEVELOPMENT OF BABY
16weeks Fetal movement can
be felt
Fine hair (lanugo) grows over the
body; foetus resembles a tiny human
being; skeleton is visible
20-22
weeks
Uterus reaches up to
the level of umbilicus
and pregnancy is
obvious
Vernix caseosa, the protective fatty
coating, begins to be deposited;
heartbeat can be heard
24
weeks
Doctor can tell where
baby's head, back
and limbs are ;
breasts have enlarged
and nipples and
areola are darker,
colostrum is
produced
Fully formed but still thin; much larger
and very active, all major organs are
working, the lungs and digestive
system need more time to develop;
body is covered in fine hair called
lanugo

18. 18
weeks
19weeks

19. 20weeks
24 weeks

20. WEEK CHANGES IN MOTHER DEVELOPMENT OF BABY
32 weeks Uterus reaches halfway
between umbilicus and
rib cage
Most babies are in a head
down position in the womb;
head is more in proportion to
the body; eyes are open;
babies born at this stage have
a good chance of living
36 weeks Weight gain is
averaging about a
pound a week; standing
and walking are
becoming very difficult
because the center of
gravity is thrown
forward
Body hair begins to disappear,
fat is being deposited
40 weeks Uterus is up to the rib
cage, causing shortness
of breath and
heartburn; sleeping is
very difficult
Not much room to move in
the womb; fully mature, baby
moves less, and the
surrounding fluid reduces and
the womb expands its limits

21. 26
weeks
36weeks

22.  The act of expelling the full term young
one from the mother’s uterus at the end
of gestation period- parturition.
 Vigorous contraction of uterus causes
delivery or expulsion of the foetus.
 Hormones responsible – oxytocin,
relaxin.

24.  Pregnancy is counted as 40 weeks,
starting from the first day of the
mother’s last menstrual cycle upto
parturition.
 During this period many
developmental changes takes
place in the foetus.

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