3.14 Pregnancy: Fertilization to Birth

(Sec 4.3 pg 117)

• The ‘simplest’ part of reproduction is bringing the sperm and the egg together. After that event, many very complex events must take place before a healthy baby is born.

• Fertilization:– Requires millions of sperm because most die on the way

and only about 100 make it as far as the egg in the oviduct.

– This starts a 9 month process. This period is divided into 3 trimesters, each three months long.

• First Trimester:– Months 1-3.– Zygote implants into the uterus and is called an embryo

by the end of the 1st week.– Sac forms around the embryo full of amniotic fluid,

which supports and protects the embryo. – By week 8 all of the organs have started to develop and

we call it a fetus. It is now about the size of a lima bean.– All nutrients are being received through the umbilical

cord that connects to the mother through the placenta.– By the end of the first trimester the fetus can move, is

about 12 cm long and around 50g, also the gender can be determined.

• Second trimester:– Months 4-6.– The fetus is growing a lot during this period. Can grow to

be as long as 30 cm during this time. – By week 24 the fetus resembles a tiny infant.

• Third Trimester:– Months 7-9.– The baby puts on most of its mass, all organ systems

now function properly.– The baby turns so that the head is pointed down in

preparation for birth.

• Birth:– Contractions start.– Cervix dilates and the baby is forced into the birth canal.– Amniotic fluid sac breaks and lubricates the birth canal.

• Stem Cells:– When the zygote first starts to develop any of its cells

could become anything, a lung cell, a brain cell, etc. – We call these cells that can become any kind of cell a

stem cell. After they become a kind of cell they can only continue to become that kind of cell. We call this differentiation.