Pregnancy, Fertilisation & Development – Year 11 Human Biology

A comprehensive set of question-and-answer flashcards covering sexual intercourse, fertilisation, early embryonic development, stem cell potency, primary germ layers, embryonic membranes, placental formation, and key developmental milestones from conception through full-term pregnancy.

What physiological change allows the penis to enter the vagina during sexual intercourse?

An erection caused by blood rushing into the erectile tissue spaces of the penis.

What is the name of the process in which rhythmic contractions propel semen out of the male urethra?

Ejaculation.

Which glands contribute the greatest volume to semen and what is one key function of their secretion?

The seminal vesicles; their thick fluid provides nourishment for the sperm.

What is insemination?

The release of semen (containing sperm) into the female vagina at the entrance to the uterus.

In which part of the female reproductive tract does fertilisation normally occur?

The fallopian (uterine) tube, about one-third of the way down from the ovary.

Name the two layers surrounding a secondary oocyte at ovulation.

The corona radiata (outer) and the zona pellucida (inner).

What reaction enables a sperm to penetrate the zona pellucida?

The acrosome reaction, in which enzymes are released to digest the glycoprotein matrix.

After a sperm enters the oocyte, what are the male and female haploid nuclei called?

Male pronucleus and female pronucleus.

What is the diploid cell formed by the fusion of male and female pronuclei?

A zygote.

Define pregnancy in human biology terms.

The period from conception to birth during which a woman carries a developing foetus in her uterus.

How long is a normal human pregnancy measured from conception?

Approximately 265 days (about 38 weeks).

What is the more common clinical length of pregnancy measured from the last menstrual period?

About 280 days or 40 weeks.

During which weeks is the developing human called an embryo?

From conception until the end of the eighth week.

What name is given to the developing human after eight weeks?

A foetus (fetus).

What is cleavage in early embryonic development?

Rapid mitotic cell divisions of the zygote resulting in smaller cells called blastomeres.

At roughly six days after fertilisation, what structure does the zygote become?

A blastocyst.

What two main parts make up a blastocyst?

An outer cell layer (trophoblast) and an inner cell mass of about 30 stem cells.

What process firmly attaches the blastocyst to the uterine wall?

Implantation into the endometrium.

Which ovarian structure secretes progesterone and oestrogen to maintain the endometrium during early pregnancy?

The corpus luteum.

Which hormone produced by the chorion peaks around the second month and supports the corpus luteum?

Human chorionic gonadotrophin (hCG).

Define totipotent stem cells and give an example.

Cells that can give rise to the entire embryo and its membranes; e.g., the zygote before formation of the inner cell mass.

What is the potency of the inner cell mass stem cells called?

Pluripotent.

What are multipotent stem cells?

Stem cells that can differentiate into a limited range of cell types within a particular tissue (e.g., blood stem cells).

Name the three primary germ layers formed during the third week.

Ectoderm, mesoderm and endoderm.

Which germ layer forms the nervous system and epidermis?

Ectoderm.

From which germ layer do skeletal muscles, bones and the heart originate?

Mesoderm.

The lining of the digestive tract and lung epithelium derive from which germ layer?

Endoderm.

List the four embryonic membranes.

Amnion, chorion, yolk sac and allantois.

What fluid fills the cavity within the amnion and give one of its functions.

Amniotic fluid; it acts as a shock absorber to protect the embryo/foetus.

Which embryonic membrane becomes the main foetal part of the placenta?

The chorion.

What structures form the core of the umbilical cord?

The yolk sac and allantois, along with foetal blood vessels.

What is the trophoblast and what two layers does it form?

The outer layer of the blastocyst; it differentiates into the cytotrophoblast (inner) and syncytiotrophoblast (outer invasive layer).

What are chorionic villi and their function?

Finger-like projections of the chorion containing foetal blood vessels; they facilitate exchange with maternal blood.

Name the three blood vessels found in the umbilical cord.

Two umbilical arteries and one umbilical vein.

What is the decidua basalis?

The maternal portion of the placenta formed from the endometrium at the implantation site.

Give two structural features of the placenta that maximise exchange efficiency.

Large surface area via villi and thin walls of villi for rapid diffusion.

What hormone produced by the placenta helps maintain pregnancy after the corpus luteum regresses?

Progesterone (also oestrogen is produced).

At what day does the embryonic heart usually begin to beat?

Around day 21.

During which week does the neural groove form, marking early CNS development?

Week 4.

By the end of the eighth week, approximately what proportion of adult body structures are established?

Over 90 %.

What is quickening and roughly when is it first felt?

The mother's perception of foetal movement; typically around 16–20 weeks.

What protective substance covers the foetal skin around 20 weeks?

Vernix caseosa.

At about which week do the testes usually descend into the scrotum in male foetuses?

Around 28 weeks.

What is considered ‘full-term’ gestation?

Approximately 38 weeks (though birth often occurs at 40 weeks by LMP dating).

Why is foetal activity reduced near 40 weeks?

Limited space in the uterus as the foetus has grown large.

Why is the foetal head proportionally smaller to the body at 40 weeks compared to earlier stages?

The body grows faster in late gestation, balancing proportions and aiding passage through the birth canal.

Explain the difference between identical and fraternal twins in terms of fertilisation.

Identical twins arise from one zygote that splits; fraternal twins come from two separate eggs fertilised by two different sperm.

Do identical twins usually share a placenta?

Yes, they usually share one placenta but have separate amniotic sacs.

Name two key respiratory changes in the foetus between 24 and 28 weeks.

Development of lung tissue and initiation of respiratory movements (practice breathing).

What foetal reflexes typically appear by the fifth month?

Gripping, sucking and swallowing reflexes.

During which trimester does the placenta become fully functional?

By the end of the first trimester (around weeks 12–13).

Which pituitary hormone rises after birth to initiate milk production?

Prolactin.

Which hormone released from the maternal pituitary around birth stimulates uterine contractions?

Oxytocin.

What structure forms the maternal-foetal gas exchange surface in the placenta?

The thin placental membrane between chorionic villi capillaries and maternal blood in intervillous spaces.

How do umbilical arteries and the umbilical vein differ in function?

Umbilical arteries carry deoxygenated blood from foetus to placenta; the umbilical vein carries oxygenated blood from placenta to foetus.

Which germ layer gives rise to the adrenal medulla and sensory receptor cells?

Ectoderm.

What major event marks the transition from pre-embryonic to embryonic stage?

Formation of the three primary germ layers (gastrulation) during week 3.

At what approximate CRL (crown-rump length) is the embryo by the end of week 8?

About 27–31 mm.

What embryonic structure becomes the future backbone?

The notochord (formed along the primitive groove).

Which weekly developmental event allows gender determination via chorionic villus sampling?

By week 8 the chromosomes are accessible for testing, allowing determination of sex.

What is the function of HCG during early pregnancy?

Maintains the corpus luteum so it continues secreting progesterone and oestrogen to sustain the endometrium.

List two reasons why the placenta acts as a selective barrier.

Its membrane allows diffusion of beneficial substances (oxygen, nutrients, antibodies) while blocking many harmful pathogens or toxins.

What substance surrounds the vessels in the umbilical cord providing protection?

Wharton's jelly (a jelly-like connective tissue).

By what week are nearly all organ systems present in rudimentary form?

By the end of week 8.

Which embryonic membrane is the first to develop and on what day does it surround the embryo?

The amnion; it encloses the embryo by about day 8 after fertilisation.

What are somites and during which weeks are they first clearly visible?

Segmented blocks of mesoderm that give rise to skeletal muscles and vertebrae; visible during weeks 4–5.

What is the main function of the yolk sac in humans?

Early nutrient transfer, gas exchange and formation of the first blood cells.

How does the placenta produce hormones to maintain pregnancy after 12 weeks?

Placental trophoblast cells synthesise progesterone and oestrogen, replacing the hormonal role of the corpus luteum.

Which primary germ layer contributes to formation of the kidneys and ureters?

Mesoderm (and some endoderm for epithelial lining).

What developmental milestone occurs at 32–34 weeks regarding positioning?

The foetus usually adopts the head-down (vertex) birth position.