Chapter 2: Prenatal
Development and
the Birth Process
Developmental Psychology A Study Notes
Session 2
Louw, D. A. & Louw, A. E. (2022). Child and adolescent development (3rd
ed. Ch. 2). Psychology Publications.
Opening Case Studies: Why the Prenatal Period
Matters
Gracie and Rosie — conjoined twins born in England in 2000, physiologically joined and
sharing one heart and a pair of lungs.
Rosie was biologically dependent on Gracie; doctors agreed the shared organs could not
sustain both long-term, and death of both was inevitable without separation.
Gracie had a strong chance of surviving separation; Rosie had no chance of surviving it.
The devoutly Catholic parents wanted the twins' fate left "in God's hands"; surgeons took
the matter to the High Court.
Legal/ethical arguments included a comparison with a mountain climber who cut the rope
holding a dangling partner, versus the view of the twins as "two persons in one body"
each with a right to live as long as possible.
The Court ordered separation against the parents' wishes; Rosie died immediately after
separation; Gracie later enjoyed good health into adulthood.
Mpho and Mphonyana — South African conjoined twins connected at the head, sharing
brain tissue; separated at Chris Hani Hospital, Johannesburg, in 1988 at 17 months old.
Mphonyana died two years after the operation; Mpho survived with mild brain damage
and hemiplegia (paralysis on one side of the body), enjoying relatively good health until
her death at 34 in 2021.
Why these cases open the chapter: they illustrate how prenatal development (before birth)
can shape postnatal development (after birth).
For many years psychologists focused only on the postnatal period; the prenatal
period/birth process were seen as having little psychological importance, partly because
the fertilisation process itself was not scientifically understood until the 18th century.
Two reasons for renewed interest today:
Research shows prenatal and birth-related factors can significantly affect later
psychological development.
Society, not just individuals, is affected — legally, ethically, religiously — as the conjoined-
twin cases show.
, Links to the new field of behaviour genetics, also called psychogenetics: the study of the
genetic basis of behaviour (Plomin, 2018).
Chapter roadmap:
(1) the prenatal period — conception, chromosomes, HGP, cell division,
dominance/recessivity, sex determination, multiple births, cloning, genetic abnormalities;
(2) the prenatal stages — germinal, embryonic, foetal;
(3) prenatal environmental influences;
(4) the birth process.
2.1 The Prenatal Period
2.1.1 The Beginnings of a New Life
Historically, people did not always link the sexual act to birth, likely owing to the long gap
between the two events (Lowder, 2013).
A widespread historical belief: every human begins life in the womb as a fully formed
"miniature person" that simply grows larger until ready for birth.
Reproductive biology belongs to embryology/genetics, but psychologists need working
knowledge of it because it underpins both normal and abnormal psychological development.
2.1.1.1 Conception
Conception (fertilisation): a sperm unites with an egg cell (ovum) in one of the two fallopian
tubes; occurs ~280 days before birth.
Egg and sperm cells = gametes / sex cells. A fertilised egg cell = zygote.
The ovum:
Largest cell in the human body — about a quarter the size of a full stop.
~400,000 egg cells present at birth, many atrophying before puberty (Augustyn, 2021);
~300,000 remain at puberty; only ~300–400 are ever ovulated in a woman's reproductive
lifetime (Cleveland Clinic, 2019).
Ovulation occurs roughly every 28 days; the egg travels along the ~10 cm fallopian tube
(about 12 hours to reach the middle), where fertilisation usually occurs.
Unfertilised egg cells break down and are expelled with blood/tissue during menstruation.
The sperm:
One of the smallest human cells; looks tadpole-like under a microscope.
Produced continuously in the testes (~300 million/day); ~100–500 million released per
ejaculation (Andrade-Rocha, 2017).
~20 million sperm must pass through the vagina for fertilisation to be likely, though only
one sperm is needed; most die from toxic vaginal acids, with only a few hundred reaching
each fallopian tube.
Sperm remain viable for about 5 days (Gargollo, 2021); an egg remains fertilisable for
~24 hours; fertilisation generally occurs within 12–36 hours (usually within 24) of the
ovum entering the tube.
Once one sperm penetrates the egg wall (leaving its tail behind), the cell wall changes so
no other sperm can enter.
Fertilisation itself: nuclei of sperm and egg unite to form a new cell with 46 chromosomes —
, the zygote, about one-twentieth the size of a pinhead.
Not all fertilised eggs become babies:
~25% of fertilised eggs are spontaneously aborted in the first few weeks, often before the
mother knows she is pregnant.
Of zygotes surviving 3–4 weeks, a further ~30% are spontaneously aborted
(miscarriage).
Fewer than half of all fertilised eggs result in a living baby (Alexander et al., 2018).
Birth-rate context: fertility rates are declining worldwide, especially in industrialised
countries (Bruni, 2019; Chamie, 2020); over 80 countries (about half the world's population)
are below the replacement rate of ~2 children per woman, while several sub-Saharan African
countries exceed 5 births per woman.
South Africa: birth registrations ≈ 2% annually (close to the world average) (Stats SA,
2017); ≈ 1 million babies born annually; registration rates vary with education and income
— lower education/income tends to associate with higher birth rates (Makiwane et al.,
2018; Samir et al., 2017).
2.1.1.2 Chromosomes, genes, DNA and RNA
Chromosomes: the zygote has 46 (23 pairs); species-specific number (mice = 20 pairs, apes =
27 pairs, peas = 7 pairs).
23 chromosomes come from each parent; every body cell has 23 pairs, but gametes
(ovum/sperm) carry only 23 chromosomes each.
Visible under a microscope as rod-shaped structures — see Figure 2.1 in the source (23
pairs illustrated; the 23rd pair is the sex chromosomes, differing in males/females).
Genes: segments of a chromosome; the smallest units of heredity, carrying hereditary
characteristics.
Estimated 20,000–25,000 genes per human cell (earlier estimates were far higher, around
100,000); precise number still uncertain because small/overlapping genes are hard to
detect.
Each gene occupies a specific chromosome position; function of most genes, and how
genes interact with each other and the environment, remains largely unknown.
DNA (deoxyribonucleic acid): the chemical basis of heredity — a double-spiral "twisted rope
ladder" molecule carrying the hereditary "recipe"/"blueprint" for cell function.
All humans are 99% genetically identical (National Human Genome Research Institute,
2020); only ~0.1% of DNA accounts for physical/psychological differences between
people.
DNA strands are so fine that ~5 million could pass through a needle's eye.
RNA (ribonucleic acid): produced by DNA since DNA itself does not leave the chromosome;
often called "messenger RNA" — carries blueprint information from the nucleus to the
cytoplasm and directs protein construction.
2.1.1.3 The Human Genome Project (HGP)
Behaviour genetics/psychogenetics is now a major Psychology study area; genetics also
matters in Medicine, Zoology, Botany.
HGP: multi-disciplinary project founded in the USA in 1990 (National Human Genome
Research Institute, 2020); "genome" = all genetic material of an organism.
Development and
the Birth Process
Developmental Psychology A Study Notes
Session 2
Louw, D. A. & Louw, A. E. (2022). Child and adolescent development (3rd
ed. Ch. 2). Psychology Publications.
Opening Case Studies: Why the Prenatal Period
Matters
Gracie and Rosie — conjoined twins born in England in 2000, physiologically joined and
sharing one heart and a pair of lungs.
Rosie was biologically dependent on Gracie; doctors agreed the shared organs could not
sustain both long-term, and death of both was inevitable without separation.
Gracie had a strong chance of surviving separation; Rosie had no chance of surviving it.
The devoutly Catholic parents wanted the twins' fate left "in God's hands"; surgeons took
the matter to the High Court.
Legal/ethical arguments included a comparison with a mountain climber who cut the rope
holding a dangling partner, versus the view of the twins as "two persons in one body"
each with a right to live as long as possible.
The Court ordered separation against the parents' wishes; Rosie died immediately after
separation; Gracie later enjoyed good health into adulthood.
Mpho and Mphonyana — South African conjoined twins connected at the head, sharing
brain tissue; separated at Chris Hani Hospital, Johannesburg, in 1988 at 17 months old.
Mphonyana died two years after the operation; Mpho survived with mild brain damage
and hemiplegia (paralysis on one side of the body), enjoying relatively good health until
her death at 34 in 2021.
Why these cases open the chapter: they illustrate how prenatal development (before birth)
can shape postnatal development (after birth).
For many years psychologists focused only on the postnatal period; the prenatal
period/birth process were seen as having little psychological importance, partly because
the fertilisation process itself was not scientifically understood until the 18th century.
Two reasons for renewed interest today:
Research shows prenatal and birth-related factors can significantly affect later
psychological development.
Society, not just individuals, is affected — legally, ethically, religiously — as the conjoined-
twin cases show.
, Links to the new field of behaviour genetics, also called psychogenetics: the study of the
genetic basis of behaviour (Plomin, 2018).
Chapter roadmap:
(1) the prenatal period — conception, chromosomes, HGP, cell division,
dominance/recessivity, sex determination, multiple births, cloning, genetic abnormalities;
(2) the prenatal stages — germinal, embryonic, foetal;
(3) prenatal environmental influences;
(4) the birth process.
2.1 The Prenatal Period
2.1.1 The Beginnings of a New Life
Historically, people did not always link the sexual act to birth, likely owing to the long gap
between the two events (Lowder, 2013).
A widespread historical belief: every human begins life in the womb as a fully formed
"miniature person" that simply grows larger until ready for birth.
Reproductive biology belongs to embryology/genetics, but psychologists need working
knowledge of it because it underpins both normal and abnormal psychological development.
2.1.1.1 Conception
Conception (fertilisation): a sperm unites with an egg cell (ovum) in one of the two fallopian
tubes; occurs ~280 days before birth.
Egg and sperm cells = gametes / sex cells. A fertilised egg cell = zygote.
The ovum:
Largest cell in the human body — about a quarter the size of a full stop.
~400,000 egg cells present at birth, many atrophying before puberty (Augustyn, 2021);
~300,000 remain at puberty; only ~300–400 are ever ovulated in a woman's reproductive
lifetime (Cleveland Clinic, 2019).
Ovulation occurs roughly every 28 days; the egg travels along the ~10 cm fallopian tube
(about 12 hours to reach the middle), where fertilisation usually occurs.
Unfertilised egg cells break down and are expelled with blood/tissue during menstruation.
The sperm:
One of the smallest human cells; looks tadpole-like under a microscope.
Produced continuously in the testes (~300 million/day); ~100–500 million released per
ejaculation (Andrade-Rocha, 2017).
~20 million sperm must pass through the vagina for fertilisation to be likely, though only
one sperm is needed; most die from toxic vaginal acids, with only a few hundred reaching
each fallopian tube.
Sperm remain viable for about 5 days (Gargollo, 2021); an egg remains fertilisable for
~24 hours; fertilisation generally occurs within 12–36 hours (usually within 24) of the
ovum entering the tube.
Once one sperm penetrates the egg wall (leaving its tail behind), the cell wall changes so
no other sperm can enter.
Fertilisation itself: nuclei of sperm and egg unite to form a new cell with 46 chromosomes —
, the zygote, about one-twentieth the size of a pinhead.
Not all fertilised eggs become babies:
~25% of fertilised eggs are spontaneously aborted in the first few weeks, often before the
mother knows she is pregnant.
Of zygotes surviving 3–4 weeks, a further ~30% are spontaneously aborted
(miscarriage).
Fewer than half of all fertilised eggs result in a living baby (Alexander et al., 2018).
Birth-rate context: fertility rates are declining worldwide, especially in industrialised
countries (Bruni, 2019; Chamie, 2020); over 80 countries (about half the world's population)
are below the replacement rate of ~2 children per woman, while several sub-Saharan African
countries exceed 5 births per woman.
South Africa: birth registrations ≈ 2% annually (close to the world average) (Stats SA,
2017); ≈ 1 million babies born annually; registration rates vary with education and income
— lower education/income tends to associate with higher birth rates (Makiwane et al.,
2018; Samir et al., 2017).
2.1.1.2 Chromosomes, genes, DNA and RNA
Chromosomes: the zygote has 46 (23 pairs); species-specific number (mice = 20 pairs, apes =
27 pairs, peas = 7 pairs).
23 chromosomes come from each parent; every body cell has 23 pairs, but gametes
(ovum/sperm) carry only 23 chromosomes each.
Visible under a microscope as rod-shaped structures — see Figure 2.1 in the source (23
pairs illustrated; the 23rd pair is the sex chromosomes, differing in males/females).
Genes: segments of a chromosome; the smallest units of heredity, carrying hereditary
characteristics.
Estimated 20,000–25,000 genes per human cell (earlier estimates were far higher, around
100,000); precise number still uncertain because small/overlapping genes are hard to
detect.
Each gene occupies a specific chromosome position; function of most genes, and how
genes interact with each other and the environment, remains largely unknown.
DNA (deoxyribonucleic acid): the chemical basis of heredity — a double-spiral "twisted rope
ladder" molecule carrying the hereditary "recipe"/"blueprint" for cell function.
All humans are 99% genetically identical (National Human Genome Research Institute,
2020); only ~0.1% of DNA accounts for physical/psychological differences between
people.
DNA strands are so fine that ~5 million could pass through a needle's eye.
RNA (ribonucleic acid): produced by DNA since DNA itself does not leave the chromosome;
often called "messenger RNA" — carries blueprint information from the nucleus to the
cytoplasm and directs protein construction.
2.1.1.3 The Human Genome Project (HGP)
Behaviour genetics/psychogenetics is now a major Psychology study area; genetics also
matters in Medicine, Zoology, Botany.
HGP: multi-disciplinary project founded in the USA in 1990 (National Human Genome
Research Institute, 2020); "genome" = all genetic material of an organism.
