Pregnancy

Pregnancy
(after conception and implantation)

Remember that fertilization happens in the fallopian tubes . . . implantation happens 7 to 9 days after fertilization
in the UTERUS (ideal conditions) . . . at 4 days following fertilization, that little mass of cells is called a
BLASTOCYST.

HUMAN BLASTOCYST, and you don't have to remember the parts listed. I
am copying for you the fantastic information given on this website.

The one-celled zygote created at conception divides by mitosis into 2 genetically
identical cells within 12 to 15 hours... and then another division to make 4 cells
within the next 12-15 hours... and so forth. By the third day following conception
the cells have divided to form a small ball that is known as a blastocyst. The
blastocyst enters the uterus and begins to implant in the blood rich lining of the
uterus.

Implantation occurs when the blastocyst sinks into the endometrium. Through
the action of hormones produced by the blastocyst, an intensive chemical
exchange occurs between the blastocyst and the mother's tissue to signal its
presence in the uterus. This exchange temporarily impairs the mother's immune
system to prevent antibodies from attacking the blastocyst as foreign matter.

During the second week after conception, the inner cell mass of the blastocyst
structure separates into three distinct layers that may be seen under a
microscope. The outer layer is known as the ectoderm, which will later form the
skin, hair, sweat glands, tooth enamel, salivary glands, and all the nervous
tissue, including the brain. The middle layer, or mesoderm, will form the muscles,
bones, blood, circulatory system, teeth, connective tissues and kidneys. The
inner layer, the endoderm, will form most of the internal organs (stomach,
intestines, liver, lungs, heart, and so on).

These three layers are differentiated by the third week following conception.
They will continue to proliferate into the various organ system tissues during the
embryonic period of prenatal development.

departments.weber.edu/.../ images/Blastocyst.gif

4 week old embryo
www.sciencemuseum.org.uk/
exhibitions/lifecycl...

An EMBRYO. The embryo is the name of the prenatal organism (baby) from 2 to
8 weeks after conception, during which time the foundations of all body
structures and organs are laid down.

By four weeks, the embryo has a head, tail, backbone and limb buds - which will eventually become arms and legs. The beginnings
of ears and eyes are also visible. Its heart is already beating (starting at day 21), and the other organs are forming fast. An umbilical
cord starts to grow between the embryo and the placenta. During this time, the embryo is especially sensitive to any drugs or
infections capable of crossing the placenta from the mother.

By the end of week 8 (the 10th week of pregnancy), all the different parts of the body are in place. Fingers and toes form - the
embryo now has unique fingerprints. Its head is still very large compared to its body - almost half its length. The brain is growing at
about 100,000 new brain cells every minute. All embryos look identical at seven weeks, before the sex-determining gene is switched
on in males.
From week 9 (the 11th week of pregnancy) the developing embryo is called a FETUS. The size of its body catches up with the
outsized head, making it look more human, although the legs are still slightly short. Protected by the amniotic fluid, the fetus can
move around and flex its limbs. By 12 weeks, it can hear sounds, and its skin is sensitive to touch.

By 20 weeks (the 22nd week of pregnancy), the mother can feel
the fetus move. It has eyebrows and hair on its head. Its entire body is
covered in fine, downy hair. A greasy substance called vernix protects
the skin. The fetus practises swallowing and digesting fluid, and can
even tell sweet tastes from bitter ones. An ultrasound scan may now
reveal whether the fetus is a boy or a girl.

The fetus at 14 weeks, National Medical Slide Bank/Wellcome Photo
Library
Below, An ultrasound scan of a 22-week-old fetus.
Giskin Day

During the second half of pregnancy, the fetus becomes
increasingly aware of the world outside. It is startled by sudden
noises, and is thought to be capable of feeling pain after 5-6
months. Rapid eye movements, associated with dreaming sleep in
adults, begin at 21 weeks (the 23rd week of pregnancy). The
fetus looks transparent, as it doesn't yet have any fat under its skin.
By 26 weeks (the 28th week of pregnancy), the lungs are ready
to breathe air. The brain has developed enough to control breathing
and body temperature to some extent. So from now on the fetus
stands a good chance of surviving even if it's born early. It has
eyelashes and fingernails. It appears less wrinkled, as more and
more fat is laid down under the skin.
The baby's skin is now pink and smooth, and its arms and legs are
fatter. The eyes have opened, and by 30 weeks (the 32nd week
of pregnancy) the pupils will dilate and contract in response to
light. The mother may feel the baby hiccuping, after it has swallowed
too much amniotic fluid. By 34 weeks, most babies have their heads
downwards, ready for birth.
The baby continues to put on weight, reaching an average of 3.4
kilograms by 38 weeks (the 40th week of pregnancy). It is
outgrowing the womb, and starts the birth off by releasing
hormones. The muscles of the mother's womb start to contract, and
labour begins. Most babies are born within ten days of their due
date - but only 5 per cent actually arrive on the predicted day.

The hands and mouth of a fetus around six months
old.
Petit Format, Nestle/Science Photo Library

A woman in her eighth month of pregnancy. Newborn baby.
Petit Format, Prof E Symonds/Science Photo Library Wellcome Photo Library

Electron micrograph of a four-day-old human embryo.
Yorgos Nikas/Wellcome Photo
Librarywww.sciencemuseum.org.uk/ exhibitions/lifecycl...

The fetus grows very quickly during weeks 13 to 16 - doubling in size
from 5 to 10 centimetres. The face starts to form, and by 14 weeks (the
16th week of pregnancy) the eyes can move. The eyelids stay closed
until the final two months of pregnancy. Now that the internal organs are
in their final positions, the bones begin to harden, although they remain
flexible until after the baby has been born.

A 16- to 20-week-old fetus.
Wellcome Photo Library