The stages of human embryo development include fertilization, cell division, differentiation, and organ formation. In the first trimester, the zygote divides into a blastula which implants in the uterus. Cells then differentiate into three layers forming the basis for organs and tissues. By the end of the first trimester, most organs have formed and the embryo is called a fetus, surrounded by amniotic sac, placenta, and umbilical cord. During development, the fetus is vulnerable to dangers like alcohol, tobacco, drugs, toxins, disease, and genetic disorders which can lead to birth defects, miscarriage, or other problems.

1. Stages of Human
Embryo Development
2017

2. The First Trimester
 Day 1
 Fertilization occurs in the fallopian tubes
 Sperm and egg unite to make one big cell called the zygote
This one cell will eventually give rise to all of the cells in the body

3.  Week 1
 Zygote Divides (one cell into two, two into four, four into eight,
and so on) into a ball of cells called a blastula, which implants
into the wall of the uterus.

5.  Month 1
 Cells continue to divide, and differentiate into a three-layered
ball of cells called a gastrula.
The outer layer of cells of the gastrula eventually differentiate more to
become the nervous system and skin. The middle layer will form the
muscle, bone, blood and connective tissue. The inner layer will form the
lining of the digestive and respiratory tract, and parts of the liver, pancreas
and many other organs.

6.  End of 1st Trimester
 Rapid growth has occurred, and most organs have now formed
 Growing embryo is now called a fetus (because most organs
have formed)

7.  Also, Placenta, umbilical cord, and
amniotic sac are formed around the
fetus
Amniotic Sac – Protective sac that
holds the fetus.
Amniotic fluid – Watery fluid inside
sac that protects the fetus.
Placenta – Site of exchange for
oxygen, nutrients, and waste (and
more) between the blood of the
fetus and the blood of the mother.
Umbilical cord – Connects the fetus
to the placenta. Bad things can also cross over the
placenta from the mother to the fetus…

8. Dangers to the Fetus
Handout
 Alcohol
 Can cause many different birth defects including organ, skeletal,
mental, and growth deformities.
 Fetal Alcohol Syndrome – a pattern of mental and physical
defects caused by the consumption of alcohol during pregnancy.

9.  Tobacco Smoke (also marijuana smoke)
 Contains toxins that can be poisonous to the fetus and to the
placenta harmful to fetus
 Reduces oxygen supply to fetus, which can lead to defects of
the heart and lungs
 Increase the risk of Sudden Infant Death Syndrome
(10% of sudden infant deaths are linked to smoking)
 Other Illegal Drugs (Cocaine, Heroin, others)
 These drugs directly affect a fetus
 The fetus can become addicted, causing it to go through
withdrawal after birth (seizures, fever, other problems)
 Increase the chances of other birth defects, miscarriage,
premature birth, and Sudden Infant Death Syndrome

10.  Environmental Toxins (pesticides, industrial
chemicals, some prescription drugs)
 Can lead to a variety of birth defects and even miscarriage.
 It depends on which toxin, and how much of it the fetus is
exposed to.
 Disease Pathogens
 Most pathogens cannot cross the placenta.
 However, some (mostly viruses) can: HIV, Herpes, Rubella,
syphilis
 Can cause miscarriage or serious birth defects
 Genetic & Developmental Disorders Disorders
 Can be random or inherited from the parents
 Examples are Heart defects, Autism, Down’s Syndrome, cystic
fibrosis, and cleft lip/palette.

12.  2nd
Trimester
 Organs continue to develop &
begin to function (heart beat,
reflexes, etc)
 3rd
Trimester
 Organs complete development,
and become full functional
 Fetus grows in size by putting
on ‘baby fat

13.  Birth! (If everything goes well…)
The amniotic sac breaks (‘water breaks’), and
a healthy baby exits through the birth canal
(head-first).
….but everything does not always go well…..
Dangers to the Fetus
Handout