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Course: UP Class 12th Biology > Unit 2

Lesson 6: Pregnancy, parturition, and lactation
Science
UP Class 12th Biology
Human reproduction
Pregnancy, parturition, and lactation
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Embryonic development review

Google Classroom
Stages of development of the fetus at the end of each trimester.
This article reviews the stages of embryonic development during human pregnancy.

Key terms

TermMeaning
blastocystearly stage zygote that gives rise to embryo
embryodifferentiated implanted zygote during early stages of development
fetusembryo after 10 weeks of pregnancy
trimesterperiod of 3 months during pregnancy
ectodermouter most germ layer
mesodermmiddle germ layer
endoderminner most germ layer

What happens during pregnancy?

Have you ever wondered why a typical human pregnancy lasts about 9 months? What goes on in there?
The embryo develops within the womb during this period. First, stem cells in the inner cell mass of the implanted blastocyst begin to differentiate. They form three germ layers that eventually give rise to all the tissues in the body - the outer ectoderm, inner endoderm, and mesoderm in the middle.
Let's take a look at what happens next.

Stages of development

A lot of changes happen very quickly both during pregnancy and after birth of the child. It's common to measure development of the fetus on a weekly basis, so let's take a look at some key timepoints.
  • By week 4: Organs start to develop, the heart forms and begins to pump blood.
  • By week 8 :The fetus develops limbs and digits by the second month
  • By week 12 (1st trimester) : Most major organs are formed, external genitalia visible.
  • By week 20 : The mother begins to feel the first movements of the fetus. Hair appears on top of the head.
  • By week 24 (2nd trimester) : The fetus has eyelashes, separated eyelids, and fine hair covering its body.
  • By week 36 (3rd trimester) : The fetus is fully developed, shifts downwards in the uterus to prepare for birth.
test your understanding
A couple who are expecting a child enter the gynecologist’s clinic for their scheduled check up. The doctor performs an ultrasound test, and points out that the baby’s arms and legs are visible for the first time.
At what stage of her pregnancy must the mother be?
Choose 1 answer:

test your understanding
After testing positive on home pregnancy kits, Nina visits the doctor for confirmation. The doctor detects a second heartbeat, affirming that Nina is pregnant.
How many weeks old must the fetus be before its heartbeat can first be detected?
Choose 1 answer:

test your understanding
There are apps that expectant mothers can use to follow the development of their babies. For example, the apps detail the size of the fetus, the features that it has formed at that stage, and so on.
Which of the following features is an app likely to predict at week 20?
Choose 1 answer:

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