Match List I with List II related to embryonic development at various months of pregnancy: List I (embryonic development) i. The foetus movement starts and hair appears on the head ii. The foetus develops limbs and digits iii. The foetus develops external genital organs iv. The foetus body is covered with fine hair; eyelids separate and eyelashes are formed List II (months of pregnancy) a. 24 weeks of pregnancy b. 20 weeks of pregnancy c. 8 weeks of pregnancy d. 12 weeks of pregnancy Choose the correct answer from the options given below:

Embryonic Development

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NCERT Class 12 · Human Reproduction · 1 NEET question

The heart forms first — by the end of the first month. Limbs and digits come by the end of the second month, and the first movements and hair on the head appear in the fifth month. NEET now asks these month-by-month milestones directly.

The idea

After implantation, the inner cell mass (embryo) differentiates into three germ layers: an outer ectoderm, a middle mesoderm and an inner endoderm, from which all tissues and organs develop. The inner cell mass contains stem cells that can give rise to all tissues and organs. Human pregnancy lasts about 9 months. After one month the embryo's heart is formed; the first sign of a growing foetus can be noticed by listening to the heart sound through a stethoscope. By the end of the second month the foetus develops limbs and digits. By the end of 12 weeks (first trimester) most major organ systems are formed, for example the limbs and external genital organs are well developed. The first movements of the foetus and the appearance of hair on the head are usually observed during the fifth month. By the end of about 24 weeks (second trimester) the body is covered with fine hair, the eyelids separate and eyelashes are formed. By the end of nine months the foetus is fully developed and ready for delivery.

Why it matters

Organs must form in a fixed order — circulation first so that every other tissue can be fed — which is why development follows a predictable timetable.

NEET lens

Memorise the NCERT timeline: 1 month — heart; end of 2nd month — limbs and digits; end of 12 weeks — major organ systems, limbs and external genital organs well developed; 5th month — first movements, hair on head; end of 24 weeks — body covered with fine hair, eyelids separate, eyelashes formed; 9 months — fully developed. Matching questions pair these with 8, 12, 20 and 24 weeks.

Clinical significance

Because most organs form in the first trimester, this is when the embryo is most sensitive to drugs, alcohol, smoking and infections such as rubella; doctors therefore advise extra care in early pregnancy. Ultrasound scans and foetal heart sounds are used to track these milestones.

Introduction

After implantation, the embryo grows through a fixed sequence of stages until birth about nine months later.

Core concept

Germ layers: inner cell mass → ectoderm, mesoderm, endoderm → all tissues and organs; the inner cell mass contains stem cells. Timeline (NCERT): after 1 month — heart formed (heart sound can be heard by stethoscope); end of 2nd month (8 weeks) — limbs and digits; end of 12 weeks (first trimester) — most major organ systems formed, limbs and external genital organs well developed; 5th month (about 20 weeks) — first foetal movements, hair on the head; end of 24 weeks (second trimester) — body covered with fine hair, eyelids separate, eyelashes formed; end of 9 months — fully developed, ready for delivery.

Go deeper

Trimester view: first trimester (0-12 weeks) = organ formation; second trimester (13-24 weeks) = movements, hair, eyelids separate; third trimester = growth and maturation. Gestation in humans is about 9 months.

From Class 10

You learnt that the embryo grows in the uterus and gets food through the placenta. NEET asks what develops when — month by month.