Human Reproduction
Senior 6 · Biology
Human Reproduction
Lesson 5 of 5
35 min
On 25 July 1978 a baby girl, Louise Brown, was born in England after being conceived outside her mother's body, the first person born by in-vitro fertilisation. Robert Edwards, who developed the method, received the 2010 Nobel Prize in Physiology or Medicine. Today, knowledge of reproductive hormones is used in two opposite directions: to help couples who cannot conceive, and to let couples decide whether and when to have children. The same knowledge also explains how sexually transmitted infections spread and how they can be prevented. This lesson ends the unit.
Definition
Infertility: a disease of the reproductive system defined by the World Health Organization as the failure to achieve a pregnancy after 12 months or more of regular unprotected sexual intercourse.
In-vitro fertilisation (IVF): fertilisation of oocytes by sperm outside the body ("in glass"), followed by transfer of an embryo to the uterus.
Family planning: the ability of individuals and couples to decide the number and spacing of their children, using contraception or by treating infertility.
Contraception: any method that prevents pregnancy.
Sexually transmitted infection (STI): an infection passed from one person to another mainly by sexual contact.
The WHO estimates that about one in six people of reproductive age worldwide experience infertility at some time in their lives. The cause may lie with the man, with the woman, or with both partners.
| Possible causes | |
|---|---|
| In men | Few or no sperm in the semen (low sperm count); sperm that swim poorly or have an abnormal shape; blocked sperm ducts, for example after an infection; problems with ejaculation; hormonal imbalance |
| In women | Failure to ovulate, often because of a hormonal imbalance; oviducts blocked or scarred, often after an untreated STI such as chlamydia or gonorrhoea; abnormalities of the uterus; fertility falling with age, especially after about 35 |
| Affecting both | Untreated STIs; smoking; heavy alcohol use; being severely overweight or underweight |
Treatments depend on the cause: drugs that act like FSH and LH can stimulate ovulation; surgery can sometimes open blocked tubes; and when other methods fail, IVF can bypass blocked oviducts or a low sperm count.
Issues to evaluate. IVF has allowed millions of couples to have children, but: success rates fall sharply as the woman's age rises; transferring more than one embryo raises the chance of twins or triplets, with greater risks to mothers and babies; the hormone treatment carries some risk to the woman; spare embryos raise ethical and religious questions about their storage, use or disposal; and the high cost means that access is unequal, which the WHO notes is a particular challenge in low- and middle-income countries.
Worked example 1: how likely is pregnancy within a year?
Illustrative value: a healthy couple who have regular unprotected intercourse have about a 20% chance of conceiving in any one cycle. Assume the cycles are independent. (a) What is the probability of not conceiving in 12 cycles? (b) Explain why the WHO's definition of infertility uses 12 months.
Family planning lets parents space births and choose the size of their family, which protects the health of mothers and children and helps families plan for each child's care and education. For young people, abstaining from sexual intercourse is the only completely reliable way to avoid both pregnancy and STIs. Methods available to adults work in two main ways: by stopping gametes being released, or by stopping sperm reaching or fertilising the oocyte.
| Method | How it works | Notes |
|---|---|---|
| Fertility awareness (calendar or natural methods) | Avoiding intercourse on the days around ovulation | No side effects, but least reliable, because the day of ovulation varies |
| Male and female condoms | A barrier that stops sperm entering the uterus | The only methods that also protect against STIs, including HIV, when used correctly every time |
| Combined pill | Synthetic oestrogen and progesterone inhibit FSH and LH secretion, so no follicle matures and there is no ovulation; also thickens cervical mucus | Very effective if taken every day; no protection against STIs |
| Progestogen injection or implant | Progesterone-like hormone prevents ovulation and thickens cervical mucus | An injection lasts about three months; an implant under the skin of the arm, several years |
| Intrauterine device (IUD) | A small device placed in the uterus by a health worker; copper is toxic to sperm and oocytes, so fertilisation does not happen | Very effective for many years |
| Sterilisation (vasectomy, tubal ligation) | Sperm ducts or oviducts are cut and tied, so gametes cannot meet | Permanent; for people who are sure they want no more children |
| Emergency contraception | Used soon after unprotected intercourse: pills mainly delay or prevent ovulation; a copper IUD prevents fertilisation | Works best when used as early as possible; not for regular use |
Health workers at health centres help each person choose a suitable method. Only condoms protect against both pregnancy and STIs.
| Infection | Cause | Main effects if untreated | Treatment |
|---|---|---|---|
| Chlamydia | Bacterium (Chlamydia trachomatis) | Often no symptoms; can scar the oviducts and cause infertility | Antibiotics |
| Gonorrhoea | Bacterium (Neisseria gonorrhoeae) | Discharge and pain on urinating; infertility in both sexes | Antibiotics (resistance is growing) |
| Syphilis | Bacterium (Treponema pallidum) | Painless sore, then rash; later damage to the heart and brain; can cross the placenta | Antibiotics |
| Genital herpes | Herpes simplex virus | Painful blisters that return from time to time | No cure; antiviral drugs reduce attacks |
| Human papillomavirus (HPV) | Virus | Some types cause cervical cancer years later | Prevented by vaccination before exposure |
| HIV | Human immunodeficiency virus, a retrovirus | Destroys helper T cells; after years, AIDS, when the immune system fails | No cure; lifelong antiretroviral therapy (ART) |
HIV. HIV infects helper T lymphocytes, the cells that coordinate the immune response, and slowly destroys them. According to the WHO, it is passed on through body fluids of a person living with HIV, including blood, breast milk, semen and vaginal secretions, and from a mother to her child during pregnancy and delivery. People cannot become infected through ordinary day-to-day contact such as kissing, hugging, shaking hands, or sharing personal objects, food or water. There is no cure, but antiretroviral therapy controls the virus: the WHO states that people living with HIV who take ART and have an undetectable viral load will not transmit HIV to their sexual partners.
Prevention of STIs and HIV:
Warning
Remember: protection and help. Any sexual contact with a child is abuse and a crime, and coercing anyone into sex is never acceptable. Rwanda Investigation Bureau numbers: 116 (reporting child abuse), 3512 (domestic and gender-based violence), 3029 (Isange One Stop Centre) and 166 (emergencies and reporting crime). After a sexual assault or any other possible exposure to HIV, go to a health facility at once: PEP must be started within 72 hours. Health centres also give confidential STI and HIV testing, treatment and advice on family planning.
Worked example 2: comparing IVF success
Illustrative results from one clinic in a year: women under 35 had 120 embryo transfers, giving 36 pregnancies; women over 40 had 40 transfers, giving 4 pregnancies. (a) Calculate each success rate. (b) Suggest a biological reason for the difference. (c) Evaluate whether these data alone justify a firm conclusion.
TED-Ed: "How in vitro fertilization (IVF) works", by Nassim Assefi and Brian A. Levine. Watch for the role of hormones in each step of IVF.
Practice: infertility, family planning and STIs
Key points
Further reading for this unit
Sources and credits
Figure 1, the tables, the worked examples (with illustrative data) and the practice questions were written by Ganzaa.
