Pregnancy: Fertilization to Birth

You have learned that the role of the female reproductive system is to produce eggs and to carry the fertilized egg from fertilization (also called conception) to birth. How exactly does fertilization occur? What changes take place in the embryo during the nine months of pregnancy? You began life as a single cell, a zygote, about 0.1 mm in diameter. One single cell divided into the trillions of very different cells that make up your body.

Fertilization Millions of sperm are released into the vagina during sexual intercourse. Most die on the way, and only about 100 make it as far as the oviduct1

These 100 or so sperm surround the egg and try to penetrate the egg's outer coating. As soon as one sperm penetrates the egg (Figure 1), the egg releases a protein that prevents other sperm from penetrating. The cell membranes of the sperm and the egg fuse. The sperm's nucleus enters the egg, where it will fuse with the egg's nucleus to produce the zygote.

Fertilization is the beginning of a nine-month (38-week) period of pregnancy, which ends with the birth of the offspring. The nine months of pregnancy

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are divided into three trimesters (three-month periods) for convenience.

First Trimester The}irsLtrime~ter s~a~t~ at ~ertilizati?n and en~s after t~e thir~~nth. . The zyg~egms div1dmg m the oviduct and 1s called a~ embry when 1t imi,l-an~to the uterus by the end of the first week. secreted that prevent menstruation from happening.

Soon after implanting, a sac forms around the embryo and is filled with amniotic fluid, which supports, protects, and maintains a warm environment for the embryo until birth. When a mother's water breaks during birth, it is this amniotic fluid that is released.

LEARNING TIP - ----, You can use a table to help you organize information for studying. Make a three-column table with the following headings: First trimester, Second trimester, Third trimester. Under the appropriate heading, record important information in point-form notes. Include the words in bold in your notes.

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Figure 1 A single sperm cell fusing with an egg cell

By the fourth week, the brain and nervous system are developing, and the heart is beating. The embryo is now 500 times bigger than the zygote, although it is still only about 5 mm long. The beginnings of eyes, ears, and nose are noticeable at the end of week five (Figure 2). All the cells of the embryo follow the instructions in their DNA and develop different structures and functions. Limb buds and a tail are visible. The tail will reduce, and all that will be left at birth is the tail bone.

Figure 2 In the late first trimester, rudimentary limbs and eyes are visible. This fetus is about 4 cm long and has a mass of less than 1 O g.

NEL 4.3 Pregnancy: Fertilization to Birth 117

amniotic fluid

fetus

Figure 3 The fetus receives nutrition from the mother via the placenta and the umbilical cord.

(~~,--Figure 4 By the end of the second trimester, facial features are present and internal systems are maturing.

Figure 5 In the third trimester, the fetus fills the uterus. With so little space, the fetus no longer moves around as much.

II Unit A Reproduction

has the beginnings of all th b yo now . e • ...1... ek the em r but it is only the s12e of a f th e1gh1±1~ ' k human, By the end o e · • e fetus loo s ther through the placenta d • all ~a fetus. fr m the mo

organs an IS c e . ,_. g nutrition ° d grows to 20 to 30 cm in . I · ow re eivm . disk an lima bean. t IS n . shaped like a th and blood vessels from the . 3) The placenta IS th mo er (Figure . . bl od vessels from e ther's blood into the fetus's di t It contains O £ the mo

aine er. diffuse rom h ' blood. Because the fetus Nutrients and oxygen . into the mot er s . . . . · fi m the fetus diffuse ach other, diffusion 1s possible. blood. Wastes ro e so close to e

th , d £ tus's blood vessels ar . ul system of the fetus. The fetus's mo er s an e the crrc atory . . Th Other's blood does not enter 1 nta through the umbilical cord. e m d to the p ace . · ulatory system is connecte 1 nd bones are formmg. The fetus circ h muse es a By the end of the third mont ' h' p The heart is beating and

d may iccu . . can flex arm and leg muscles, an . and gender can be determined.

. ns are formmg, 1 blood is circulatmg. 'Sex orga b d The fetus is about 12 cm ong, The head is almost half the size of the o y.

with a mass of around 50 g. . b t I kg she may not look different. , · · mass is a ou ' . Since the mothers gam m h r such as no menstruation, . f gnancy, oweve ,

There are usually signs O

pre A blood test done by a health-care d fmes nausea. . enlarged breasts, an some

1 The fetus is most susceptible to birth

provider can confirm a p~egnancy. . defect is a structural or defects during the first tnmeSter. A birth f; b. th physiological problem in the fetus that develops be ore Ir . . .•

The Second Trimester Months four, five, and six are a time of intense growth for the fetus. The fetus

from 8 to 30 cm long by the end of the sixth month. The heartbeat can grows . . . be detected with a stethoscope. The fetus is sucking and swallowmg ammotic fluid, which will be excreted in urine. The mother now looks pregnant, as her abd~men swells. Movement of the fetus can be felt by the mother, first as a flutter but soon as pokes and kicks as the legs and arms develop.

At 24 weeks, the fetus resembles a tiny infant (Figure 4). Fingers and toes have elongated. Facial features are more developed, and eyelashes are visible. The fetus is now covered with a fine hair. Practice breathing also occurs, although no gas exchange occurs. Oxygen reaches the fetus through the blood vessels in the umbilical cord.

The Third Trimester During the last three months (Figure 5), the fetus puts on most of its mass as it continues to develop in preparation for birth. The organ systems are functioning properly. The fetus has sleeping and waking patterns and reacts to stimuli from the outside world. The brain is developing rapidly. The bones are developed but still soft. The eyelids are open. The fetus fills the uterus and the mother's belly may undulate as the baby changes position. During the last month, the fetal hair disappears. The fetus turns so that the head is pointing down in preparation for brrth. The lungs contmue to mature right up to birth. At 3 7 weeks, the fetus is considered full term. The size of the fetus ranges from 45 to 55 cm and can be up to 4.5 kg in mass, although the average size is 53 cm and 3.4 kg at birth.

Jirth \.bout nine months or 266 days after fertilization, the muscles of the mother's 1terus contract regularly (every 15 to 20 minutes, and lasting at least 40 seconds). [he birth process, called labour, has begun. Hormones control the process, .vhich is described in Figure 6.

Figure 6 The birth process

(1) The cervix begins to diiate, or open up. The membrane surrounding tpe baby is forced into the vagina (now called the birth canal) .

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I Q The amniotic membrane breaks, and amniotic fluid lubricates the canal. This is called the "breaking of the water." '

. G Once the cervix has widened to 10_cm, u:erine contractions push the baby's head into the

· birth canal. The baby's hea€1 is followed by the rest of the body.

G) When the head and shoulders are free of the birth canal, the rest of the baby slips out easily. The baby is born. A short while later, the placenta, often called the "afterbirth," is pushed out of the uterus. Once the baby is breathing on his or her own, the umbilical cord is cut and tied off to prevent bleeding. The "belly button" is the scar from the umbilical cord.

Did You " KNow r All Pregnancies Are Not Equal Pregnancy is also called gestation. The gestation period is often related to the size of the mammal. Here are some mammals and their gestation periods in weeks: • elephant-88 • horse-48 • grizzly bear-30 • lion-17 • wolf-9 • squirrel-3 .5

LEARNING TIP ----------, Imagine the bii:th process (Figure 6) and the explanation's (A to D) happening visually. Share your mental images with a partner.

NEL 4.3 Pregnancy: Fertilization to Birth

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Stem Cells and Differentiation . · s in its DNA to produce the mor

The one-celled zygote has the mstructwn ,a t· t" • th e · body Dm.eren ta ton 1s e than 200 specialized types of cells m your · diffi t ·a1 · 1· d cells into many eren spec1 ized process of growing from unspec1a 1ze

• • I · ·tis a ball of roughly 150 cells. After the zygote d1v1des severa times, 1 . . _ · 1· d 11 (F" 7) Thi·s ball has stem cells m 1t. Stem cells are unspec1a 1ze ce s 1gure . · 1· d 11 th th the ability to reproduce themselves. When sten-. unspec1a 1ze ce s a ave . ·••

300X 11 d 11 d . · · the dau·ghter cells will either remam stem cells or ce s un ergo ce 1v1s10n, . .

begin differentiating into one of many different types of specialized c:11~. Figure 7 The zygote has divided several times. This ball of cells contains stem cells, which are unspecialized. As these cells divide, they can differentiate into all the different types of cells in the body.

As the embryo grows and develops into a fetus, and these stem cells d1v1de, they Jose their ability to grow into all the different types of cells. These can then only differentiate into certain families of tissues. As the f~tus develops further, stem cells are found in only certain tissues. Stem cells m these tissues can only form into the types of cells found in these tissues. For example, the stem cells found in the bone marrow can differentiate into the more than a dozen types of blood cells.

If you would like to learn more about stem cells, go to

• www.science.nelson.com

Figure 8 About one in 250 births produce identical twins.

Unit A Reproduction

Stem cells are of great interest to scientists because of their ability to differentiate into different types of cells. Scientists might be able to use this property of stem cells to develop new technologies. For example, scientists might be able to grow new heart muscle cells to replace cells that have been damaged in heart attacks. In order to do research, however, scientists need a source of stem cells. Stem cells can be found in certain adult tissues, such as bone marrow, and in the blood in the umbilical cord of a newborn. Another possible source is the extra embryos produced after in vitro fertilization (You will learn about in vitro fertilization in humans in Section 4.7.) Many eggs are fertilized, but only a few embryos are implanted. The extra embryos are frozen for future use. There is much controversy around stem cell research, and legislation in Canada controls the use of extra embryos for research . •

Twins Occasionally, more than one fetus develops inside the uterus. There are two types of twins: identical twins and fraternal twins. Fraternal twins occur when two eggs are released at the same time, usually one from each ovary. Since there are plenty of sperm cells in the oviducts, both eggs are fertilized. Each zygote develops its own placenta. Fraternal twins have the same birth date, but they are as similar or different as any brothers and sisters. They have different genetic material.

Identical twins (Figure 8) result from one fertilized egg that splits at the zygote or early embryo stage. Normally, the zygote undergoes mitosis to become a ball of cells, which stay together and become an embryo. Occasionally, when the zygote undergoes cell division, the daughter cells separate from each other and each daughter cell develops into an embryo. However, they share the same placenta. These twins contain identical genetic material because they came from the same egg and sperm. Identical twins are the same gender because they contain identical DNA.

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(l;HECK YOUR Understanding

\ 1. How large is a fertilized egg?

2. I (a) How many sperm cells fertilize an egg? I (b) How many sperm cells actually reach an egg?

')-(c) What happens to the sperm that do not reach an egg?

/ 3, Describe_ what happens in the egg after sperm Y penetration.

t 4. What is another name for the fertilized egg? ' \ s. Where in the female reproductive system does

fertilization take place?

y 6. What event happens that changes a zygote into I an embryo?

I 7. Identify structures A to E in Figure 9.

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Figure 9

_.:::;,;~~ ~ -A

B C D

E

d \,, · · fl 'd< y- 8~ What are the functions of the amniotic m . 9. Where does the embryo get its nutrition before ry the umbilical cord forms?

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V 10. Women do not menstruate once the embryo has implanted. What controls this?

\ 11. How are the nine months of pregnancy divided? .I

12. During which trimester does each of the y, following events occur? (a) Eyelids have lashes. (b) Heart begins beating. ( c) Activity decreases. ( d) Size of the head is almost half of the fetus. ( e) Embryo becomes a fetus. (f) Placenta develops. (g) -Fetus responds to stimulus. (h) All organs are formed. (i) All organ systems are working. (j) Fetus swallows amniotic fluid. (k) Arm and leg buds form. (1) Mother can feel fetal movement.

~3. What functions do the placenta and the umbilical cord serve? Is the zygote a stem cell? Explain.

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\ 15. About how many different types of cells are found in your body?

"VL6. \ 17.

What two signs signal the beginning of birth? Which part of the baby normally leaves the birth canal first?

\ 18. What happens to the umbilical cord and the placenta once the baby is born? .

19. Copy Table 1 into your notebook and use it to :7 compare the two types of twins.

Table 1

Characteristic Fraternal twins Identical twins.

Number of eggs fertilized Placenta Genetic material

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NEL 4.3 Pregnancy: Fertilization to Birth 121 ,.,