NCERT Solutions for Class 9th Science Chapter 11 .5.7 Pregnancy and childbirth — Pause and Ponder

Book page 22211 Updated on2026-09-08

Q6.

Ravi suddenly notices that he is growing taller rapidly, his shoulders are broadening, and his voice cracks. What stage of life is he entering?

Answer

Ravi is entering puberty — the stage of adolescence at which the body becomes reproductively mature. All three changes he has noticed are classic signs of it.

What Ravi notices	What is happening inside
Rapid gain in height	The growth spurt — the long bones lengthen quickly under the influence of hormones
Shoulders broadening	Bone and muscle build up in the shoulder girdle, one of the physical changes the chapter says the testes' hormone brings about in boys
Voice cracking	The larynx (voice box) enlarges and the vocal cords lengthen and thicken, so the pitch of the voice drops

Why the voice "cracks" rather than simply deepening: the vocal cords grow longer over a few months, but the muscles that control their tension take time to relearn how to work with the new length. During that period the pitch slips unpredictably between the old high register and the new low one. Once the growth is complete and the control is re-learned, the cracking stops.

What is driving all of it: the testes now produce a hormone — a chemical that regulates functions elsewhere in the body. The chapter says it controls sperm production and causes the physical changes seen in boys during puberty. So the same signal that starts sperm formation is producing the height, the shoulders and the voice.

Tip: the chapter is careful to add a point Ravi should hear. Sexual maturity is not the same as emotional maturity. The body becomes capable of reproduction gradually, but handling feelings, communicating clearly and making thoughtful decisions takes longer to develop.

Q7.

Rina’s period occurs every 28 days. Her last period was on the 5th of March. On which day is she most likely to get her next period?

Answer

On 2 April.

Day 1 of the cycle = 5 March
next period begins on day 1 of the next cycle = 5 March + 28 days
days left in March after the 5th = 31 − 5 = 26
28 − 26 = 2 → the count runs 2 days into April
next period = 2 April

Rina's 28-day cycle laid out on a calendar. Day 1 is the first day of bleeding, so the next period starts on day 1 of the following cycle — 28 days later.

Why the count starts on the first day of bleeding: a cycle is counted from day 1 of one period to day 1 of the next, not from the day the bleeding stops. So the 28 days are added to 5 March, giving 2 April as the next day 1.

Tip: "most likely" is doing real work in this question. The chapter says the cycle repeats typically every 21 – 35 days, and even in the same person it varies with illness, stress, travel and weight change. 2 April is the best estimate, not a guarantee — a point that returns in exercise Question 13.

Q8.

A human zygote has just formed. How many chromosomes does it have?

Answer

46 — the full human number, restored the instant the two gametes fuse.

sperm (haploid) = 23 chromosomes
egg (haploid) = 23 chromosomes
zygote = 23 + 23 = 46 chromosomes = 23 pairs

Meiosis halves the chromosome number, fertilisation restores it. This is why the number stays at 46 generation after generation instead of doubling.

Why the halving must come first: if each gamete carried the full 46, the zygote would have 92, the next generation 184, and so on. Meiosis prevents this by separating the members of each pair so that a gamete receives only one chromosome from each pair. The chapter states the outcome exactly: in humans, cells have 46 chromosomes but sperm and eggs have only 23, so that when they combine the zygote has the same number as the parents.

Tip: the zygote's 46 are not a random 46. They are 23 matched pairs — one member of each pair from the father and the other from the mother. That pairing is the whole basis of the variation counted in Activity 11.4.

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