Pregnancy: From Fertilization to Birth

BC Science Probe 9Section 4.3

Pages 117-121

Pregnancy

• So females produce eggs and males produce sperm… but how does the rest happen?

Fertilization

• During sexual intercourse, millions of sperm are released into the vagina.– Most will die on the way– Only about 100 will actually make it to the oviduct

where they can try to penetrate the egg.

Fertilization

• To prevent more than one sperm getting in, the egg releases a special protein the prevents the others as soon as one gets through.

• Then the single sperm’s nucleus fuses with the egg’s nucleus to produce a zygote.

• This is the beginning of a (approximately) 9-month pregnancy.

First Trimester

• Fertilization – end of month 3.• The zygote begins dividing in the oviduct and

is called an embryo when it implants in the uterus but the end of week 1.

• Hormones are secreted that prevent menstruation from happening.

First Trimester

• After implantation, a sac forms around the embryo and is filled with amniotic fluid.– Supports– Protects– Keeps warm

First Trimester

• By week 4 – The brain and nervous system are developing.– The heart is beating– The embryo is now 500 times bigger than the zygote (it’s

now about 5mm long)

First Trimester

• Week 5– Eyes nose and ears start to show up.– Limb buds and tail are visible.• The tail will reduce by the time the baby is born and all that will be left is the tail bone.

First Trimester

• Week 8– The embryo has the beginnings of all the organs

and is now called a fetus.– It looks human, but is about the size of a lima

bean.

First Trimester

• The fetus gets its nutrition from the mother through the placenta.– It has blood vessels from both the mother and the

baby– Nutrients and oxygen are brought to the baby

from the mother.– Wastes are taken away from the baby.– The placenta is connected to the fetus by the

umbilical cord.

First Trimester

• Month 3– Muscles and bones forming– Fetus can flex and may hiccup– Heart is beating– Sex organs are forming– About 12 cm long and 50 g

First Trimester

• Signs of pregnancy in the first trimester:– No menstruation– Enlarged breasts– Nausea– Hormones in a blood test– Very little weight gain

Second Trimester

• The fetus will be about 30 cm long by the end of the 6th month.

• It grows a lot during the second trimester• The fetus will be able to kick

Second Trimester

• Week 24– The fetus looks like a baby, but still tiny– Fingers and toes can be seen– Eyelashes have developed– The fetus is covered in fine hair

Third Trimester

• Most of the mass gets put on in the last three months.

• Organ systems are functioning.• The fetus sleeps and wakes and reacts to the

outside world.

Third Trimester

• Brain is developing rapidly.• Bones are developed but still soft.• Eyelids are open.• Fetal hair will disappear in the last month.

Third Trimester

• By 37 weeks, the fetus is considered full-term and ready to be born.

Birth

• Controlled by hormones, the muscles in the mother’s uterus contract regularly (labour) to get ready to push the baby out.

• This happens approximately 9 months after fertilization.

Birth

• The cervix begins to dilate.• The membrane

surrounding the baby is forced into the vagina (now called the birth canal).

Birth

• The amniotic membrane breaks and amniotic fluid lubricates the canal.

• The is called “water breaking”.

Birth

• Once the cervix has widened to 10 cm, uterine contractions push the baby’s head into the birth canal.

• The baby’s head is followed by the rest of the body.

Birth

• When the head and shoulders are free, the rest of the baby slips out and the baby is born!

• A while later that placenta is pushed out also.• Once the baby is breathing on her own, the

umbilical cord is cut and tied. (Your belly button is your umbilical cord scar.)

Stem Cells and Differentiation

• The DNA in the zygote has all of the instructions to make all of the specialized cells in a body.– This is called differentiation when unspecialized

cells become specialized.

Stem Cells and Differentiation

• Stem cells are unspecialized cells that can reproduce themselves.

• When these cells undergo cell division, the daughter cells either remain stem cells, or begin differentiating.

Stem Cells and Differentiation

• As the fetus grows and develops, the stem cells lose their ability to differentiate into all types of tissues.

• Only certain tissues keep making stem cells and even then, they can only form into different types of that tissue.– For example: the stem cells found in bone marrow

can differentiate into different types of blood cells.

Stem Cells and Differentiation

• Stem cells are continually being researched to develop new technologies.

• As a source for stem cells for research, the blood from the umbilical cord of a newborn can be used.

Twins

• Fraternal Twins– Two eggs get released at

the same time.• Usually one from each ovary.

– Both eggs get fertilized.– Each zygote develops its

own placenta.– They have the same

birthday, but are as similar or different as any brothers or sisters.

Twins

• Identical Twins– Come from one fertilized egg.– The egg splits at the zygote or early embryo stage.– They share the same placenta.– These twins have the same DNA because they

came from the same eggs and sperm.