By Carly and Katie
First Trimester: 0-12 weeks
• Fertilization occurs in the fallopian tube, it involves the joining of a
sperm cell with a secondary oocyte. The secondary oocyte
develops into a fertilized ovum, or zygote. The zygote is
composed of 46 chromosomes, 23 from the sperm cells and 23
from the oocyte.
• The sperm cells travel through the cervix into the uterus, although
they do not all reach fallopian tubes. It takes approximately 3-5
days for the fertilized ovum to travel from the fallopian tube to
• Cell division, or cleavage, occurs and the zygote gets smaller.
The zygote develops into a fluid filled structure known as the
• In the uterus the blastocyst attaches to the wall of the
endometrium, this is referred to as implantation.
Zygote, Embryo, Fetus
• 4 days after fertilization, the zygote becomes an
• The embryo remains an embryo for 8 weeks, then it will
be referred to as a fetus.
• The ‘yolk sac’ is another key element which acts as the
circulatory system for the embryo.
3 Layers of the Fetus
• The fetus is composed of 3 layers:
• Ectoderm (outer) – skin, hair, finger nails, brain, retina…
• Mesoderm (middle) – muscles, blood vessels and blood, reproductive
• Endoderm (inner) – liver, pancreas, thyroid, parathyroid, urinary bladder…
Development in First Trimester
• After four weeks, a complex vascular network has formed inside
of the placenta. The placenta is the site in which gas and nutrient
exchange occurs between a mother and her baby. The umbilical
chord also serves as an exchange site for nutrients.
• By the end of the first month, the 1 cm long embryo is 500x larger
than the fertilized egg. Many important organs/systems have
begun to develop, such as the heart, brain and tiny fingers and
toes. By the 9th week sucking reflex is evident and arms and legs
are beginning to move and the mother can feel these kicks.
Second Trimester: 14-20 weeks
Development in Second Trimester
• In the second trimester the fetus, or baby, is about 8 cm long. All
of the organs have formed, not all fully developed yet, and the
fetus gradually increases size.
• The fetus is starting to look more like a human, and kicks even
more in the womb. Soft hair covers the body, and six months into
the pregnancy the baby develops eyelids with lashes and bone
Third Trimester: 28-40 weeks
Development in Third Trimester
•In the third trimester, the baby is growing rapidly.
The body mass is getting bigger, and the organs
continue to enlarge.