Pregnancy and Embryonic Development
BiologyHuman ReproductionFor NEET aspirants
Pregnancy and embryonic development follow implantation: the placenta forms, the embryo grows into a foetus over about nine months, and the baby is delivered and fed. This page covers chorionic villi and the placenta, the hormones of pregnancy, the germ layers and stem cells, month-wise development, parturition and lactation, as in the NCERT Class 12 chapter Human Reproduction. NEET questions on pregnancy and embryonic development often ask the pregnancy-only hormones, the foetal ejection reflex with oxytocin, and the role of colostrum.
On this page1Placenta2Hormones of pregnancy3Germ layers4Month-wise growth5Parturition6Lactation7Exam essentials8Quick revision9Solved examples10Practice
Key Points at a Glance
- After implantation, chorionic villi appear on the trophoblast, surrounded by uterine tissue and maternal blood.
- ★ Must learn Placenta: chorionic villi + uterine tissue, interdigitated; the structural and functional unit between foetus and mother.
- Placenta: supplies oxygen and nutrients, removes carbon dioxide and wastes; linked to the embryo by the umbilical cord.
- ★ Must learn Placental hormones: hCG, hPL, estrogens, progestogens; relaxin comes from the ovary late in pregnancy.
- ★ Must learn hCG, hPL and relaxin are produced in women only during pregnancy.
- Inner cell mass → ectoderm and endoderm, then mesoderm between them; it contains stem cells.
- Heart at 1 month; limbs and digits by 2 months; major organs by 12 weeks; movements and head hair in month 5; eyelashes by 24 weeks.
- ★ Must learn Parturition: foetus and placenta signal → foetal ejection reflex → oxytocin from the maternal pituitary → stronger contractions.
- Gestation period: about 9 months; the placenta is expelled soon after the baby.
- ★ Must learn Colostrum: milk of the first few days of lactation, rich in antibodies for the new-born.
1. Placenta
- After implantation, finger-like projections called chorionic villi appear on the trophoblast.
- The chorionic villi are surrounded by the uterine tissue and maternal blood.
- The chorionic villi and the uterine tissue become interdigitated (interlocked like fingers) with each other.
- Together they form the placenta: a structural and functional unit between the developing embryo (foetus) and the maternal body.
1.1 Functions of the placenta
- ★ Exam imp Supplies oxygen and nutrients to the embryo.
- Removes carbon dioxide and excretory or waste materials produced by the embryo.
- Is connected to the embryo through the umbilical cord, which helps transport substances to and from the embryo.
- Acts as an endocrine tissue and produces several hormones.
★ Very important Placenta: the chorionic villi of the trophoblast interdigitated with the uterine tissue. It is both the exchange organ between mother and foetus and an endocrine tissue.
Memory Trick
Two partners make the placenta: Chorionic villi from the Child's side (trophoblast) and uterine tissue from the mother's side. Child + mother, interlocked.
Key idea
Chorionic villi + uterine tissue = placenta: it feeds the foetus, removes its wastes and makes hormones.
2. Hormones of Pregnancy
| Hormone | Source | Point to remember |
|---|---|---|
| hCG (human chorionic gonadotropin) | Placenta | Produced only during pregnancy |
| hPL (human placental lactogen) | Placenta | Produced only during pregnancy |
| Estrogens, progestogens | Placenta | Levels increase several-fold in maternal blood |
| Relaxin | Ovary, in the later phase of pregnancy | Produced only during pregnancy |
| Cortisol, prolactin, thyroxine | Adrenal (cortisol), pituitary (prolactin), thyroid (thyroxine) | Levels increase several-fold in maternal blood |
- ★ Exam imp hCG, hPL and relaxin are produced in women only during pregnancy.
- During pregnancy, the levels of estrogens, progestogens, cortisol, prolactin, thyroxine and others rise several-fold in maternal blood.
- These raised hormones are essential for supporting fetal growth, metabolic changes in the mother and maintenance of pregnancy.
Memory Trick
Pregnancy-only hormones: two h's and an R = hCG, hPL and Relaxin.
Hormones raised several-fold: Estrogens, Progestogens, Cortisol, Prolactin, Thyroxine = Every Pregnant Cow Produces Twins.
Hormones raised several-fold: Estrogens, Progestogens, Cortisol, Prolactin, Thyroxine = Every Pregnant Cow Produces Twins.
NEET Focus
Relaxin is secreted by the ovary in the later phase of pregnancy, not by the placenta. Estrogens and progestogens are not pregnancy-only hormones; they are only raised several-fold. Statement questions often swap these.
Quick Recall: tap to check
Name the three hormones produced in women only during pregnancy.
hCG, hPL and relaxin.
Which hormone of pregnancy is secreted by the ovary?
Relaxin, in the later phase of pregnancy.
Name two hormones, not made by the placenta, whose levels rise several-fold in maternal blood during pregnancy.
Any two of cortisol, prolactin and thyroxine.
Key idea
The placenta makes hCG, hPL, estrogens and progestogens; the ovary adds relaxin; several other hormones rise many times.
3. Germ Layers and Stem Cells
- Immediately after implantation, the inner cell mass (embryo) differentiates into an outer ectoderm and an inner endoderm.
- A mesoderm soon appears between the ectoderm and the endoderm.
- These three layers give rise to all tissues (organs) in adults.
- ★ Exam imp The inner cell mass contains stem cells, which have the potency to give rise to all the tissues and organs.
Memory Trick
Ecto out, Endo in, Meso in the middle. The mesoderm is the last to appear, squeezed in between the first two.
Key idea
Inner cell mass → ectoderm and endoderm → mesoderm between them; stem cells can form every tissue.
4. Development of the Foetus Month by Month
- Human pregnancy lasts 9 months.
| Time in pregnancy | Development |
|---|---|
| After one month | The embryo's heart is formed. The heart sound, heard through a stethoscope, is the first sign of a growing foetus. |
| End of the second month | The foetus develops limbs and digits. |
| End of 12 weeks (first trimester) | Most major organ systems are formed; the limbs and external genital organs are well developed. |
| During the fifth month | The first movements of the foetus and the appearance of hair on the head are usually observed. |
| End of about 24 weeks (second trimester) | The body is covered with fine hair, the eyelids separate and eyelashes form. |
| End of nine months | The foetus is fully developed and ready for delivery. |
Memory Trick
Milestones 1, 2, 3, 5, 6, 9 months: Happy Little Ones Move Like Babies = Heart (1), Limbs and digits (2), Organ systems (3, that is 12 weeks), Movements and head hair (5), Lashes and eyelids (6, that is 24 weeks), Birth (9).
★ Very important The first sign of a growing foetus is its heart sound, heard through a stethoscope; the heart forms after one month. The first movements are usually observed only in the fifth month.
Extra Depth: Pregnancy in other mammals: about 2 months in dogs and cats, and about 22 months in elephants.
Key idea
Heart at one month, organs by 12 weeks, movements in month 5, eyelashes by 24 weeks, full development by 9 months.
5. Parturition
- Gestation period: the average duration of human pregnancy, about 9 months.
- Vigorous contraction of the uterus at the end of pregnancy causes expulsion (delivery) of the foetus.
- Parturition: the process of delivery of the foetus (childbirth).
- ★ Exam imp Parturition is induced by a complex neuroendocrine mechanism involving cortisol, estrogens and oxytocin.
Steps of parturition:
- Signals for parturition originate from the fully developed foetus and the placenta.
- These signals induce mild uterine contractions called the foetal ejection reflex.
- The reflex triggers the release of oxytocin from the maternal pituitary.
- Oxytocin acts on the uterine muscle and causes stronger uterine contractions.
- The stronger contractions stimulate further secretion of oxytocin; this stimulatory reflex continues, giving stronger and stronger contractions.
- The baby is expelled out of the uterus through the birth canal: parturition.
- Soon after the infant is delivered, the placenta is also expelled out of the uterus.
★ Very important The foetal ejection reflex triggers oxytocin release from the maternal pituitary, and oxytocin in turn causes stronger uterine contractions. This is why doctors inject oxytocin to induce delivery.
Tips and Tricks
Read the loop as a chain that feeds itself: contraction → oxytocin → stronger contraction → more oxytocin. Note the two sources: the signal starts in the foetus and placenta, but oxytocin comes from the mother's pituitary.
Memory Trick
Parturition order: Signal, Reflex, Oxytocin, Contractions, Expulsion of baby, then Placenta = Small Rains Often Cause Early Puddles.
Quick Recall: tap to check
What is the foetal ejection reflex?
Mild uterine contractions induced by signals from the fully developed foetus and the placenta.
From where is oxytocin released during parturition?
From the maternal pituitary.
Which hormone do doctors inject to induce delivery?
Oxytocin.
Key idea
Foetus and placenta signal → foetal ejection reflex → maternal oxytocin → stronger contractions → baby, then placenta, delivered.
6. Lactation
- The mammary glands differentiate during pregnancy.
- They start producing milk towards the end of pregnancy; this process is lactation.
- Lactation helps the mother feed the new-born during the first few months of growth.
- ★ Exam imp Colostrum: the milk produced during the initial few days of lactation.
- Colostrum contains several antibodies, absolutely essential to develop resistance in new-born babies.
- Doctors recommend breast-feeding during the initial period of infant growth for bringing up a healthy baby.
Key idea
Mammary glands differentiate in pregnancy; colostrum, the first milk, gives the new-born antibodies.
7. Exam Essentials
Pairs to Match
| Term | Feature or role |
|---|---|
| Chorionic villi | Finger-like projections on the trophoblast |
| Placenta | Structural and functional unit between foetus and mother |
| Umbilical cord | Connects the placenta to the embryo |
| hCG and hPL | Placental hormones, only during pregnancy |
| Relaxin | Secreted by the ovary in the later phase of pregnancy |
| Cortisol, prolactin, thyroxine | Raised several-fold in maternal blood |
| Stem cells | In the inner cell mass; can form all tissues and organs |
| Mesoderm | Appears between the ectoderm and the endoderm |
| One month | Embryo's heart formed |
| End of 12 weeks | Most major organ systems formed (first trimester) |
| Fifth month | First foetal movements; hair on the head |
| End of 24 weeks | Fine body hair, eyelids separate, eyelashes |
| Foetal ejection reflex | Mild uterine contractions from foetal and placental signals |
| Oxytocin | From the maternal pituitary; stronger uterine contractions |
| Colostrum | Antibodies for the new-born |
Exceptions
- Relaxin comes from the ovary, not the placenta.
- Only hCG, hPL and relaxin are pregnancy-only hormones; estrogens and progestogens are only raised.
- The signals for parturition come from the foetus and placenta, but oxytocin comes from the maternal pituitary.
- The placenta is expelled after the baby, not before.
- The mesoderm appears last, after the ectoderm and endoderm.
- Milk production begins towards the end of pregnancy, not only after childbirth.
- The first foetal movements come only in the fifth month, although the heart forms after the first month.
Numbers to Remember
- Gestation period: about 9 months.
- Heart formed: after 1 month. Limbs and digits: by the end of 2 months.
- First trimester ends: 12 weeks. Second trimester ends: about 24 weeks.
- First movements and head hair: fifth month.
- Germ layers: 3 (ectoderm, mesoderm, endoderm).
- Pregnancy-only hormones: 3 (hCG, hPL, relaxin).
- Colostrum: the initial few days of lactation.
8. Quick Revision
- Chorionic villi on the trophoblast + uterine tissue, interdigitated = placenta.
- Placenta: oxygen and nutrients in; carbon dioxide and wastes out; umbilical cord links it to the embryo.
- Placenta is an endocrine tissue: hCG, hPL, estrogens, progestogens.
- Relaxin from the ovary in late pregnancy; hCG, hPL and relaxin occur only in pregnancy.
- Estrogens, progestogens, cortisol, prolactin, thyroxine rise several-fold: fetal growth, maternal metabolism, maintenance of pregnancy.
- Inner cell mass → ectoderm + endoderm → mesoderm between them; stem cells can form all tissues.
- Heart after 1 month (heart sound = first sign); limbs and digits by 2 months.
- 12 weeks (first trimester): major organs; fifth month: movements and head hair.
- 24 weeks (second trimester): fine hair, eyelids separate, eyelashes; 9 months: ready for delivery.
- Gestation about 9 months; parturition by a neuroendocrine mechanism (cortisol, estrogens, oxytocin).
- Foetus and placenta → foetal ejection reflex → maternal oxytocin → stronger contractions, in a loop.
- Baby through the birth canal, then the placenta is expelled.
- Lactation begins towards the end of pregnancy; colostrum gives antibodies; breast-feeding is recommended.
9. Solved Examples
Solved Example 1
Match List I with List II.
List I: A. hCG, B. Relaxin, C. Oxytocin, D. Colostrum
List II: I. Antibodies for the new-born, II. Maternal pituitary, III. Placenta, IV. Ovary in late pregnancy
Choose the correct answer:
(A) A-III, B-IV, C-II, D-I
(B) A-IV, B-III, C-II, D-I
(C) A-III, B-II, C-IV, D-I
(D) A-III, B-IV, C-I, D-II
List I: A. hCG, B. Relaxin, C. Oxytocin, D. Colostrum
List II: I. Antibodies for the new-born, II. Maternal pituitary, III. Placenta, IV. Ovary in late pregnancy
Choose the correct answer:
(A) A-III, B-IV, C-II, D-I
(B) A-IV, B-III, C-II, D-I
(C) A-III, B-II, C-IV, D-I
(D) A-III, B-IV, C-I, D-II
Solution:
Answer: (A). hCG is made by the placenta (III); relaxin by the ovary in late pregnancy (IV); oxytocin is released from the maternal pituitary (II); colostrum carries antibodies (I).
Solved Example 2
Read the statements about the placenta.
A. It is formed by chorionic villi and uterine tissue.
B. It removes carbon dioxide and wastes produced by the embryo.
C. It secretes relaxin in the later phase of pregnancy.
D. It is connected to the embryo through the umbilical cord.
E. It acts as an endocrine tissue.
Choose the correct answer:
(A) A, B and C only
(B) A, B, D and E only
(C) B, C and E only
(D) A, C, D and E only
A. It is formed by chorionic villi and uterine tissue.
B. It removes carbon dioxide and wastes produced by the embryo.
C. It secretes relaxin in the later phase of pregnancy.
D. It is connected to the embryo through the umbilical cord.
E. It acts as an endocrine tissue.
Choose the correct answer:
(A) A, B and C only
(B) A, B, D and E only
(C) B, C and E only
(D) A, C, D and E only
Solution:
Answer: (B). C is wrong: relaxin is secreted by the ovary. A, B, D and E are correct.
Solved Example 3
Arrange the events of parturition in the correct order.
A. Release of oxytocin from the maternal pituitary
B. Signals from the fully developed foetus and placenta
C. Expulsion of the placenta
D. Foetal ejection reflex
E. Expulsion of the baby
Choose the correct answer:
(A) B, A, D, E, C
(B) D, B, A, E, C
(C) B, D, A, E, C
(D) B, D, A, C, E
A. Release of oxytocin from the maternal pituitary
B. Signals from the fully developed foetus and placenta
C. Expulsion of the placenta
D. Foetal ejection reflex
E. Expulsion of the baby
Choose the correct answer:
(A) B, A, D, E, C
(B) D, B, A, E, C
(C) B, D, A, E, C
(D) B, D, A, C, E
Solution:
Answer: (C). Signals (B) → foetal ejection reflex (D) → oxytocin (A) → stronger contractions expel the baby (E) → the placenta is expelled (C).
Solved Example 4
Which of the following hormones is NOT produced only during pregnancy?
(A) hCG
(B) hPL
(C) Relaxin
(D) Prolactin
(A) hCG
(B) hPL
(C) Relaxin
(D) Prolactin
Solution:
Answer: (D). Prolactin levels only rise several-fold during pregnancy; hCG, hPL and relaxin are produced only during pregnancy.
Solved Example 5
Statement I: The first movements of the foetus are usually observed during the fifth month of pregnancy.
Statement II: By the end of 24 weeks, the eyelids separate and the eyelashes are formed.
(A) Both Statement I and Statement II are correct
(B) Statement I is correct but Statement II is incorrect
(C) Statement I is incorrect but Statement II is correct
(D) Both Statement I and Statement II are incorrect
Statement II: By the end of 24 weeks, the eyelids separate and the eyelashes are formed.
(A) Both Statement I and Statement II are correct
(B) Statement I is correct but Statement II is incorrect
(C) Statement I is incorrect but Statement II is correct
(D) Both Statement I and Statement II are incorrect
Solution:
Answer: (A). Both match the timeline: movements and head hair in month five; fine hair, separated eyelids and eyelashes by 24 weeks.
Solved Example 6
By the end of the first trimester (12 weeks) of human pregnancy:
(A) The heart is just beginning to form
(B) Most major organ systems are formed
(C) Eyelashes are formed
(D) The foetus is ready for delivery
(A) The heart is just beginning to form
(B) Most major organ systems are formed
(C) Eyelashes are formed
(D) The foetus is ready for delivery
Solution:
Answer: (B). The heart forms after one month; eyelashes form by 24 weeks; the foetus is ready only at nine months.
10. Practice Questions
Practice Questions
- Match List I with List II.
List I: A. After one month, B. End of second month, C. Fifth month, D. End of nine months
List II: I. Limbs and digits, II. Fully developed foetus, III. Heart formed, IV. First movements and hair on the head
Choose the correct answer:
(A) A-III, B-I, C-IV, D-II
(B) A-I, B-III, C-IV, D-II
(C) A-III, B-IV, C-I, D-II
(D) A-III, B-I, C-II, D-IVAnswer: (A). One month-heart; two months-limbs and digits; fifth month-movements and hair; nine months-ready for delivery. - Read the statements.
A. The inner cell mass first differentiates into ectoderm and endoderm.
B. The mesoderm appears outside the ectoderm.
C. Stem cells of the inner cell mass can give rise to all tissues and organs.
D. The three germ layers give rise to all adult tissues.
E. The trophoblast differentiates into the three germ layers.
Choose the correct answer:
(A) A, B and C only
(B) A, C and D only
(C) B, D and E only
(D) A, C, D and E onlyAnswer: (B). The mesoderm appears between the ectoderm and endoderm (B wrong); the inner cell mass, not the trophoblast, forms the germ layers (E wrong). - Arrange the formation of the placenta in order: A. Interdigitation of villi with uterine tissue, B. Implantation, C. Chorionic villi appear on the trophoblast, D. Placenta forms.
(A) B, C, A, D
(B) C, B, A, D
(C) B, A, C, D
(D) B, C, D, AAnswer: (A). Implantation → chorionic villi → interdigitation → placenta. - Which statement about parturition is NOT correct?
(A) It is induced by a neuroendocrine mechanism
(B) The signals originate from the foetus and the placenta
(C) Oxytocin is released from the foetal pituitary
(D) The placenta is expelled soon after the babyAnswer: (C). Oxytocin is released from the maternal pituitary. - Statement I: Colostrum is the milk produced during the initial few days of lactation.
Statement II: Colostrum contains antibodies that help the new-born develop resistance.
(A) Both Statement I and Statement II are correct
(B) Statement I is correct but Statement II is incorrect
(C) Statement I is incorrect but Statement II is correct
(D) Both Statement I and Statement II are incorrectAnswer: (A). Both are correct. - Fill in the blank: the structure that provides the vascular connection between the foetus and the uterus is called the ____.Answer: Placenta.
- What is parturition? Which hormones are involved in its induction?Answer: Parturition is the delivery of the fully developed foetus (childbirth) by vigorous uterine contractions. It is induced by a neuroendocrine mechanism involving cortisol, estrogens and oxytocin; oxytocin from the maternal pituitary causes the strong contractions.
Common Mistakes to Avoid
Watch out
- Writing that relaxin is secreted by the placenta. Correct: it is secreted by the ovary in the later phase of pregnancy.
- Listing estrogens among the pregnancy-only hormones. Correct: hCG, hPL and relaxin are pregnancy-only; estrogens only rise.
- Saying oxytocin comes from the foetus. Correct: the foetus and placenta send the signals; oxytocin comes from the maternal pituitary.
- Placing the first foetal movements in the second month. Correct: they are usually observed during the fifth month.
- Saying the trophoblast forms the germ layers. Correct: the inner cell mass forms the ectoderm, endoderm and then the mesoderm.
- Writing that the placenta is expelled before the baby. Correct: it is expelled soon after the infant is delivered.
- Defining colostrum as milk of the last months of lactation. Correct: it is the milk of the initial few days, rich in antibodies.
- Saying the placenta is formed by the embryo alone. Correct: it is formed by chorionic villi together with the uterine tissue.
Frequently Asked Questions
How is the placenta formed?
After implantation, finger-like projections called chorionic villi appear on the trophoblast. They are surrounded by uterine tissue and maternal blood. The chorionic villi and the uterine tissue become interdigitated and jointly form the placenta, the structural and functional unit between the developing foetus and the mother.
What are the functions of the placenta?
The placenta supplies oxygen and nutrients to the embryo and removes carbon dioxide and excretory wastes produced by it. It is joined to the embryo by the umbilical cord. It also acts as an endocrine tissue, producing hCG, hPL, estrogens and progestogens.
Which hormones are produced only during pregnancy?
Human chorionic gonadotropin (hCG), human placental lactogen (hPL) and relaxin are produced in women only during pregnancy. hCG and hPL come from the placenta, while relaxin is secreted by the ovary in the later phase of pregnancy. Estrogens and progestogens are produced at other times too.
What happens to the inner cell mass after implantation?
Immediately after implantation, the inner cell mass differentiates into an outer ectoderm and an inner endoderm, and a mesoderm soon appears between them. These three germ layers give rise to all adult tissues and organs. The inner cell mass also contains stem cells that can form every tissue.
What are the main stages of foetal development in humans?
The heart forms after one month, and limbs and digits by the end of the second month. Most major organ systems form by 12 weeks. First movements and head hair appear in the fifth month. By 24 weeks the eyelids separate and eyelashes form, and by nine months the foetus is ready for delivery.
What is the foetal ejection reflex?
At the end of pregnancy, signals from the fully developed foetus and the placenta induce mild uterine contractions called the foetal ejection reflex. This reflex triggers the release of oxytocin from the maternal pituitary. Oxytocin causes stronger contractions, which release more oxytocin, until the baby is expelled.
Why do doctors inject oxytocin to induce delivery?
Oxytocin acts on the uterine muscle and causes strong uterine contractions. These contractions stimulate further oxytocin secretion, and the loop continues with stronger and stronger contractions that expel the baby through the birth canal. So an injection of oxytocin can start or speed up delivery.
Why is colostrum important for a new-born baby?
Colostrum is the milk produced during the initial few days of lactation. It contains several antibodies that are absolutely essential for the new-born to develop resistance. This is one reason doctors recommend breast-feeding during the initial period of infant growth.
Previous year questions on Pregnancy and Embryonic Development
4 questions from past papers, each with a step-by-step solution.
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