Childrens Illustrated Encyclopedia

REPRODUCTION

435

Lining of uterus

breaks down and

passes out of the

vagina during

menstruation, or

a period.

Lining starts to

thicken again in

preparation for

next egg. Next

egg begins to

ripen in ovary.

Ripe egg is

released from

ovary. Egg can

be fertilized for

up to 24 hours in

Fallopian tube.

Egg reaches

uterus and

implants if

fertilized, or

breaks down

if not fertilized.

1st week 2nd week 3rd week 4th week

Premature

babies

if a baby is

born before the

37th week of

pregnancy it is

called premature

and may have

difficulty breathing.

the baby is placed

in an incubator and

monitored very

carefully until it is

strong enough to

breathe for itself.

birth

birth is the process that ends

pregnancy and pushes a baby out of

the uterus, usually after 38-40 weeks

of pregnancy. When the baby has

reached full term (left), it is about

20 in (50 cm) long. Labor is

triggered by the hormone oxytocin

and by changes in the level of other

hormones in the mother’s blood.

During labor, the cervix widens to

allow birth to take place, and powerful

contractions in the uterus push the

baby out through the vagina, usually

headfirst. if a baby is born feet first, it is

called a breech birth. the baby then takes

its first breaths and the umbilical cord is

cut. the placenta is expelled from the

uterus a few minutes later as afterbirth.

A doctor checks

the heartbeat of a

premature baby in

its incubator.

12 Weeks

its cells actively

multiplying, the

fetus continues to grow and

develop rapidly. Features such as

fingernails, toenails, and eyelids are

now visible. the baby is about 5 in

(13 cm) long. there are still 28

weeks to go before it is born.

8 Weeks

the baby is about 1 in

(25 mm) long, and all the

major parts of the body have

formed—even the fingers

and toes. the developing

baby is now called a fetus.

5 Weeks

the developing

baby is now about

(^1) ⁄ 2 in (10 mm) long.
it has a recognizable head,
back, and heart, and the
beginnings of a mouth
and eyes. the limbs are
forming as small buds. at
this stage, the developing
baby is called an embryo.
Pregnancy
about one week after fertilization, the now hollow ball of cells
embeds itself in the blood-rich lining of the uterus where it
absorbs nutrients. the cells continue to divide and change,
forming the first body tissues, such as blood vessels and
nerves. gradually, the ball of cells folds and twists into the
basic body shape of the baby. meanwhile, other cells form
the placenta, a saucer-shaped organ, in the lining
of the uterus. the placenta is fed with blood
from the mother, and oxygen and nutrients
pass to the baby through the umbilical cord.
this lifeline consists of three blood vessels;
the largest vein carries nutrients and
oxygen-rich blood to the baby,
and the smaller ones carry
waste and blood low in
oxygen back to the placenta.
Puberty
babies and children have sex organs, but they are not
able to release egg or sperm cells. at puberty, which
generally starts when people are between 10 and 15
years old, chemicals called sex hormones are released
into the bloodstream from hormonal glands. these sex
hormones cause the sex organs to mature (become fully
developed). Other changes occur at this time too,
particularly a spurt in growth.
in a boy, the testes produce a sex hormone called
testosterone. this makes hair grow on the face and
body. it also makes the voice deeper, encourages muscle
development, and sets off the production of sperm.
in a girl, the ovaries produce progesterone and
estrogen, which cause the breasts to develop and fatty
tissue to form, giving the body a more rounded shape.
From puberty onward, a woman’s body also undergoes
a monthly process called the menstrual cycle, as shown
below. changing levels of hormones thicken the uterus
lining and enrich it with blood, which will nourish a
fertilized egg if it implants.
animals
human body
Find out more
US_435_Reproduction_2.indd 435 22/01/16 4:47 pm