Conception slideshow: From egg to embryo

Colour-enhanced view of a sperm cell penetrating an egg.

An egg being released from a mature follicle on the surface of the ovary.

Sperm and egg meet inside the fallopian tube.

During their journey, many sperm run out of energy before reaching the egg.

The moment of conception.

(left) Fertilised egg undergoing cell division; (right) cells continue to divide.

A fertilised egg implanted high on the endometrial wall.

Pregnancy tests detect a pregnancy hormone.

A foetus at eight weeks of pregnancy.

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Conception: From egg to embryo

At the moment when a lone sperm penetrates a mature egg, conception or fertilisation takes place. To understand better the incredible process of conception, take a journey with us from tiny egg to growing embryo.

Ovulation

Each month, a mature egg is released from one of the woman's two ovaries – this is called ovulation. Ovulation takes place about two weeks after the first day of the last menstrual period. The rare picture shown here is the clearest ever taken of human ovulation. Observing ovulation in humans is extremely rare, and previous images have been fuzzy.

Passage into the fallopian tube

Once the egg is released from the ovary, it travels into the fallopian tube where it remains until a single sperm penetrates it during fertilisation.

The laborious journey of the sperm

An average ejaculate discharges 40-150 million sperm which eagerly swim upstream towards the fallopian tubes on their mission to fertilise an egg. Fast-swimming sperm can reach the egg in half an hour, while others may take days. The sperm can live for 48-72 hours. Only a few hundred will even come close to the egg, owing to the many natural barriers and hurdles that exist in the female reproductive tract.

Fertilisation: Sperm penetrates egg

If a sperm cell meets and penetrates an egg, it will fertilise the egg. The fertilisation process takes about 24 hours. When fertilisation happens, changes occur on the surface of the egg to prevent other sperm from penetrating it. At the moment of fertilisation, the genetic makeup is complete, including the sex of the infant.

The cells begin to divide

The fertilised egg begins dividing rapidly, growing into many cells. It leaves the fallopian tube and enters the uterus three to four days after fertilisation. Rarely, the fertilised egg does not leave the fallopian tube; this is called an ectopic pregnancy and is a danger to the mother.

Implantation

After entering the uterus, the fertilised egg attaches to the uterine lining, or endometrium. This process is called implantation. The cells continue to divide.

Pregnancy hormones

Human chorionic gonadotropin (hCG) is a hormone present in the blood within about a week of conception. It is the hormone detected in a blood or urine pregnancy test, but it usually takes three to four weeks for levels of hCG to be high enough to be detected by pregnancy tests. It is secreted by cells that develop into the placenta.

Foetal development

After implantation, some cells become the placenta while others become the embryo. About three weeks after ovulation, the baby's brain, spinal cord, heart and others organs begin to form. The heart begins beating during week five. During week seven, the umbilical cord appears. At the eighth week the developing baby, now called a foetus, is well over half an inch long – and growing. A 'full term' delivery generally occurs around 40 weeks.