An Unborn Baby Eats, Breathes and Excretes Through an Organ Left Behind at Birth

A single fertilised cell, smaller than a grain of sand, becomes a baby in about nine months. During that time it cannot eat, breathe or get rid of wastes by itself. Everything it needs passes through one temporary organ, the placenta, while it floats safely in fluid inside the uterus.

This part covers the key stages of pregnancy, the placenta, the foetal membranes and amniotic fluid, and the two kinds of twins.

Fertilisation is the fusion of a sperm with an ovum in the fallopian tube, implantation is the embedding of the young embryo in the uterine wall, gestation is the time the baby develops in the uterus, and parturition is the birth of the baby.

- Fertilisation — the nucleus of a sperm fuses with the nucleus of the ovum, forming a zygote with $46$ chromosomes; it happens in the upper part of the fallopian tube
- Implantation — the zygote divides repeatedly as it moves down the tube, and about a week later the ball of cells sinks into the thick lining of the uterus
- Gestation — the period of development in the uterus, about $280$ days in humans
- Parturition — birth; strong contractions of the uterus, stimulated by oxytocin, push the baby out through the cervix and vagina

Worked example. Doctors count gestation as about $40$ weeks:

$$40\times 7=280\text{ days}$$

An everyday example. When a family is told a baby's expected due date, the doctor has counted about $40$ weeks from the first day of the mother's last period.

The substance. After about two months, once the main organs have formed, the embryo is called a foetus.

The placenta forms from finger-like projections of the embryo's outer membrane growing into the uterine wall, and across its thin barrier food and oxygen pass from the mother's blood to the foetus while carbon dioxide and urea pass back.

Formation and structure:

- Villi, finger-like projections from the embryo's outer membrane, grow into the uterine wall
- They sit in spaces filled with the mother's blood, giving a very large surface for exchange
- The placenta is a disc-shaped organ joined to the foetus by the umbilical cord, which carries the foetus's blood vessels
- The blood of mother and foetus do not mix; substances cross thin membranes mainly by diffusion

Roles of the placenta:

- Nutrition — glucose, amino acids, vitamins and minerals pass to the foetus
- Respiration — oxygen passes to the foetus; carbon dioxide passes to the mother
- Excretion — urea and other wastes pass to the mother's blood, and her kidneys remove them
- Protection — some antibodies cross, giving the newborn short-term protection

An everyday example. Pregnant women are commonly given iron and folic acid tablets, because the growing foetus draws these nutrients from the mother's blood through the placenta.

The boundary case. The placenta is not a perfect filter: alcohol, nicotine, some medicines and some viruses can cross it and harm the foetus.

The placenta secretes hormones that keep the pregnancy going, the foetal membranes enclose and connect the foetus, and the amniotic fluid cushions it and lets it move freely.

Endocrine function of the placenta:

- Human chorionic gonadotropin (hCG) — keeps the corpus luteum active early in pregnancy
- Progesterone and oestrogen — taken over from the ovary later, keeping the uterine lining in place and preventing further ovulation

Foetal membranes:

- Amnion — the innermost membrane, enclosing the fluid-filled amniotic cavity
- Chorion — the outer membrane, which forms the foetal part of the placenta
- Allantois — contributes blood vessels to the umbilical cord

Functions of amniotic fluid:

- Acts as a shock absorber, protecting the foetus from jolts
- Keeps the temperature even
- Lets the foetus move and prevents it sticking to the membranes
- Helps at birth, when the membranes break and the fluid lubricates the passage

An everyday example. When a pregnant woman's water breaks, the amnion has burst and amniotic fluid is released — a sign that birth is near.

The substance. A home pregnancy test works because hCG is made only once a pregnancy has begun.

Identical twins come from one fertilised ovum that splits into two, so they share the same genes, while fraternal twins come from two separate ova fertilised by two different sperms, so they are no more alike than ordinary siblings.

Identical (monozygotic) twins:

- One sperm fertilises one ovum, and the early embryo splits into two
- Same genes, so always the same sex and very similar in appearance
- Often share one placenta

Fraternal (dizygotic) twins:

- Two ova are released and each is fertilised by a different sperm
- Different genes, like any brother and sister
- May be the same sex or different sexes
- Each has its own placenta

Worked example. For fraternal twins, each baby is equally likely to be a boy or a girl. The possible pairs are boy–boy, boy–girl, girl–boy, girl–girl, so:

$$P(\text{same sex})=\frac{2}{4}=\frac{1}{2}$$

An everyday example. If twins in your class are a boy and a girl, they must be fraternal, since identical twins are always the same sex.

The boundary case. Identical twins still have different fingerprints, because fine details form during development, not from genes alone.