The Reproductive Systems & Conception

The Female Reproductive System

The female reproductive system is designed to produce eggs, receive sperm, provide an environment for fertilisation, and support a developing baby throughout pregnancy. The main structures are the ovaries, the fallopian tubes, the uterus and the cervix. Each ovary contains hundreds of thousands of immature eggs from birth. Once puberty begins, hormones trigger a monthly cycle in which one egg matures and is released.

The uterus (womb) is a thick-walled muscular organ about the size of a closed fist. Its inner lining, the endometrium, thickens each month in preparation for a possible pregnancy. If no fertilised egg arrives, this lining is shed during menstruation. The cervix is the narrow lower entrance to the uterus; it is normally kept closed by a mucus plug but softens and dilates during labour.

The Menstrual Cycle

The menstrual cycle averages 28 days but varies between 21 and 35 days in different individuals. It is controlled by four hormones: follicle-stimulating hormone (FSH) triggers an egg to mature inside a follicle in the ovary; oestrogen causes the uterine lining to thicken; luteinising hormone (LH) triggers ovulation (the release of the mature egg); and progesterone maintains the thickened lining after ovulation. If fertilisation does not occur, progesterone and oestrogen levels fall, the lining breaks down, and menstruation (a period) begins.

Ovulation typically occurs around day 14 of a 28-day cycle. The released egg travels along the fallopian tube towards the uterus and remains viable for fertilisation for approximately 12 to 24 hours. Sperm, by contrast, can survive in the female reproductive tract for up to five days, which means the fertile window around ovulation spans roughly six days.

The Male Reproductive System

The male reproductive system is responsible for producing, storing and delivering sperm. The testes are located in the scrotum, slightly outside the body cavity, because sperm production requires a temperature about 2°C lower than normal body temperature. Sperm are produced continuously from puberty, with around 1,500 sperm made every second in a healthy adult male. Each sperm cell carries genetic information from the father and has a head containing the nucleus, a midpiece packed with mitochondria for energy, and a tail (flagellum) for propulsion.

Mature sperm are stored in the epididymis before passing through the vas deferens. During ejaculation, sperm mix with fluids from the seminal vesicles and prostate gland to form semen. This fluid nourishes sperm and creates a slightly alkaline environment that protects them from the acidity of the vagina. A single ejaculation typically contains 200 to 500 million sperm, although only a few hundred will reach the fallopian tubes.

From Zygote to Implantation

After fertilisation, the zygote immediately begins to divide by a process called cleavage. Over the next three to four days it travels along the fallopian tube, dividing repeatedly to form a morula (a solid ball of cells) and then a blastocyst (a hollow ball with a fluid-filled cavity). By around day 6 to 10 after fertilisation, the blastocyst reaches the uterus and burrows into the endometrium in a process called implantation. Successful implantation triggers the release of human chorionic gonadotrophin (hCG), the hormone detected by home pregnancy tests, which prevents the uterine lining from shedding and signals to the body that pregnancy has begun.