Pregnancy from fertilization to birth
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Pregnancy: From Fertilization to Birth. BC Science Probe 9 Section 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.

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Pregnancy: From Fertilization to Birth

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Pregnancy: From Fertilization to Birth

BC Science Probe 9

Section 4.3

Pages 117-121


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


  • 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.


  • 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.


  • 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.


  • The cervix begins to dilate.

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


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

  • The is called “water



  • 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.


  • 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.


  • 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.


  • 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.

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