Pregnancy: From Fertilization to Birth

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. • 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.

Pregnancy: From Fertilization to Birth