Flashcards in Human Reproduction - Pregnancy Deck (20):
How Does Pregnancy Occur?
When male gametes (spermatozoa) are deposited into
a woman’s vagina, they must first move towards the
cervix, then the uterus and then to the fallopian tubes.
Remember that there will only be one ovum in only
one of the fallopian tubes.
Which Sperm Fertilizes the Egg?
Of the several million spermatozoa released, only a
few thousand will reach the ovum. Only one spermatozoon will succeed in fertilizing the
What is a Zygote?
When the spermatozoon unites with the ovum
(fertilization) it produces a zygote.
The zygote will move from the fallopian tube to the
uterus. As the zygote travels, it undergoes a series of cell
divisions. When the zygote reaches the uterus it is already a
clump of about 16 cells.
What is an Embryo?
Once the zygote has developed this far and has taken
the shape of a fluid-filled ball it will become an
From Embryo to Fetus?
By the fourth week, the embryo has grown to 500 times its initial size. The period of time before birth lasts 38-40 weeks and is known as gestation. Gestation can be divided into three trimesters.
The embryo takes on a recognizable human shape. The placenta (an organ that carries nutrients and oxygen from the mother to the fetus through the
First Trimester – Key Aspects
▶ Week 4 – The brain, heart, limbs, eyes and spinal cord begin to form. It is 1cm long.
▶ Week 8 – The embryo is now called a fetus. The first bone cells are made. The fetus has arms and legs but no fingers or toes. It is 3cm long.
▶ Week 12 – The liver, stomach, brain and heart are forming. The sex can be determined. The fetus can now
move. It is 8-10cm long.
The fetus goes through a period of growth and maturation. The mother can now feel her baby moving.
Second Trimester – Key Aspects
▶ Week 16 – The skeleton begins to form. Most organs are present but not fully developed. The fetus is about 16cm long.
▶ Week 20 – Hair has begun to grow. Buds of teeth appear. It begins to hear sounds from outside the womb. It can suck its thumb. The fetus begins using its digestive system. It
measures 25-30cm long.
▶ Week 24 – The fetus has lungs but can still not breathe on its own. It develops fingerprints. The fetus jumps when it hears a sudden noise. It is 27-35cm long.
The fetus takes up all the space in the uterine cavity.
Third Trimester – Key Aspects
▶ Week 28 – The immune system develops. Bones begin to harden. The fetus can open its eyes. It measures about 38cm long.
▶ Week 32 – The lungs and brain continue to develop. It settles into a head-down position to prepare for birth. It is about 42cm long and grows about 1cm a week.
▶ Week 36 – It can distinguish between light and dark. It recognizes its mother’s voice. It often gets hiccups. Its nails and hair continue to grow. It measures about 50cm.
The first trimester is the most critical stage of embryonic development. Certain substances such as cigarette smoke, alcohol and drugs can interfere with the normal development of the fetus.
Smoking and Pregnancy
Cigarette smoke can prevent the fetus from getting enough oxygen. This can affect it’s growth and development.
Alcohol and Pregnancy
▶ Alcohol can impair the function of the brain and
nervous system of the fetus, as well as its physical
▶ Alcohol actually stays longer in the blood of a fetus
than it does in the mother.
Drugs and Pregnancy
▶ Traces of hazardous substances, such
as certain medications, have been
found in fetal blood.
▶ These substances can cause physical
defects or mental illness.
▶ It is important to tell your doctor
before he writes you a prescription
that you are pregnant.
Pregnancy ends during weeks 38-40.
There are three steps to birth.
3) Delivery of the placenta
Oxytocin (a hormone) causes uterine contractions and
causes the cervix to open and dilate. This stage of
labor lasts 2-20 hours.
▶ Uterine contractions become very strong. The baby
moves from the uterus through the cervix and into the
▶ As the baby starts to move into the vagina, its head
rotates to help its body pass more easily.
▶ This stage lasts from 30 minutes to 2 hours.