To understand about conception and pregnancy, it helps to know about the male and female sexual organs. This information is useful in pregnancy too, when you want to ask questions and be clear about what you are told.

Man's sexual organs

Bladder

Vas deferens - This tube carries sperm from the testes to the prostate and other glands. These glands add secretions which are ejaculated along with the sperm.

Urethra - The urethra is a tube running down the length of the penis from the bladder, through the prostate gland to an opening at the tip of the penis. Sperm travel down the urethra to be ejaculated.

Penis -The penis is made of erectile tissue. This tissue acts like a sponge and, when it becomes filled with blood, the penis becomes hard and erect.

Prostate gland

Scrotum - This is the bag of skin which hangs outside the body and contains the testes. It helps to keep the testes at a constant temperature, just below the temperature of the rest of the body. This is necessary for sperm to be produced. In heat, the scrotum hangs down, away from the body, to keep the testes cool. When it is cold, the scrotum draws up closer to the body for warmth.

Testes - There are two testes. These are where sperm are made.

Woman's sexual organs

Pelvis - The baby will pass through the pelvis when he or she is born.

Womb or uterus - The womb is about the size and shape of a small upside down pear. It is made of muscle. It grows in size as the baby grows.

Fallopian tubes - These lead from the ovaries to the womb.

Ovaries - There are two ovaries, each about the size of an almond. They produce the eggs, or ova.

Cervix - This is the neck of the womb. It is normally all but closed, with just a small opening through which blood passes during the monthly period.

Vagina - The vagina is a tube about 8 cm (3 ins) long. It leads from the cervix down to the vulva, where it opens between the legs. The vagina is very elastic so it can easily stretch around a man's penis, or around a baby during labour.

Woman's monthly cycle

1. Each month a ripe egg or ovum (occasionally two) is released from one of the ovaries. This is called ovulation. The 'fingers' at the end of the fallopian tube help to direct the egg down into the tube. At the same time, the lining of the womb begins to thicken and the mucus in the cervix becomes thinner so that sperm can swim through it more easily.
2. The ripe egg begins to travel down the fallopian tube. It is here that it may be fertilised by a man's sperm if a couple have intercourse at this time. By now the lining of the womb is thick enough for the egg, if it is fertilised, to be implanted in it.
3. If the egg is not fertilised by a sperm, it passes out of the body through the vagina. It is so small that it cannot be seen. The lining of the womb is also shed in the monthly period of bleeding.

Conception

1. A woman is most likely to conceive just after the time when she ovulates - when an egg has been released from one of her ovaries. During sexual intercourse, sperm are ejaculated from a man's penis into the woman's vagina. In one ejaculation, there may be more than 300 million sperm.
2. Most of the sperm leak out of the vagina again, but some begin to swim up through the cervix. At the time of ovulation the mucus in the cervix is thinner than usual to let the sperm pass through more easily. The sperm swim into the womb and so into the fallopian tube. One sperm may then join with the egg and fertilise it. Conception is said to have taken place.
3. During the week after fertilisation, the fertilised egg, or embryo, moves slowly down the fallopian tube and into the womb. It is already growing. The embryo attaches itself firmly to the specially thickened womb lining. Hormones released by the embryo
and by the woman's ovary prevent shedding of the womb lining. The woman 'misses' her period.

A sperm is about 1/25th of a millimetre long and has a head, neck and tail. The tail moves from side to side so that the sperm can swim up the vagina into the womb and fallopian tubes.

One ripe egg or ovum (occasionally two) is released from one of the woman's ovaries every month. It moves down into the fallopian tube where it may be fertilised by a man's sperm.

Hormones

Hormones are chemicals which circulate in the blood of both men and women. They carry messages to different parts of the body, regulating certain activities and causing certain changes to take place. The female hormones, which include oestrogen and progesterone, control many of the events of the monthly cycle such as the release of the egg from the ovary and the thickening of the womb lining.

Once conception has occurred, the amount of oestrogen and progesterone increases. This causes the womb lining to build up, the blood supply to the womb and breasts to increase, and the muscles of the womb to relax to make room for the growing baby.

Heredity

Every normal human cell contains 46 chromosomes, except for the male sperm and female eggs. They contain 23 chromosomes each.

When the sperm fuses with the egg and fertilisation takes place, the 23 chromosomes from the father pair with the 23 from the mother, making 46 in all.

The chromosomes are tiny thread-like structures which each carry about 2000 genes. It is the genes that determine the baby's inherited characteristics, such as hair and eye colour, blood group, height and build.

The fertilised egg contains one sex chromosome from the mother and one from the father. The sex chromosome from the mother's egg is always the same and is known as the X chromosome. But the sex chromosome from the father's sperm may be an X or a Y chromosome.

If the egg is fertilised by a sperm containing an X chromosome, the baby will be a girl (XX). If the sperm contains a Y chromosome, then the baby will be a boy (XY).

The best time to get pregnant

An egg lives for about 12 to 24 hours after it is released from the ovary. If conception is to take place it must be fertilised within this time. Sperm can live for several days inside the woman's body. If you make love a day or so before ovulation, the sperm will have time to travel up the fallopian tubes and will be waiting when the egg is released. So the chances are highest if you make love on the day before ovulation (see chart).

Twins

Identical twins are the result of one fertilised egg splitting into two separate cells. Each cell grows into a baby. Because they originally came from the same cell, the babies have the same genes - they are the same sex and look very like each other. Non-identical twins are common. They are one result of two eggs being fertilised by two sperm at the same time. The babies may not be the same sex and will probably look no more alike than any other brothers and sisters.

Twins happen about once in every 80 pregnancies. A couple is more likely to have twins if there are twins in the woman's family. Triplets are much more rare and quads rarer still, although nowadays the use of drugs in the treatment of infertility has made multiple births more common. You may suspect that you are carrying twins if you are very sick in early pregnancy, seem bigger than your 'dates', they run in your family or you have had fertility treatment. It is usually possible to find out by about the end of the second month of your pregnancy. An ultrasound scan is needed (look here) to make the diagnosis at this stage. You should be told at this point whether the babies are in separate sacs or offered a further scan to determine this. Twin and other multiple pregnancies (e.g. triplets) carry a higher risk of most of the complications associated with pregnancy, particularly premature birth. You will probably be encouraged to have regular scans during your pregnancy to check the babies' growth. You may be advised to have a Caesarean section. Discuss this with your doctor. You can still breastfeed with twins. With triplets, or more, this may be more difficult.

It is a good idea to contact support groups like TAMBA (Twins and Multiple Births Association) and the Multiple Births Foundation (details can be found here) before the babies are born.

The signs of pregnancy

The earliest and most reliable sign of pregnancy, for women who have a regular monthly cycle, is a missed period. Sometimes women who are pregnant have a very light period, losing only a little blood. Other signs of pregnancy are listed below.

• Feeling sick - you may feel sick, or even be sick, not necessarily in the morning, but at any time. If you are being sick all the time and can't keep anything down, tell your doctor.
• Changes in your breasts - often the breasts become larger and feel tender, rather as they may do before a period. They may tingle. The veins may show up more and the nipples may darken and stand out.
• Needing to pass water more often. You may find that you have to get up in the night to do so.
• Being constipated.
• An increased vaginal discharge without any soreness or irritation.
• Feeling tired.
• Having a strange taste in your mouth - many women describe it as metallic.
• 'Going off' certain things like tea or coffee, tobacco smoke or fatty food, for example.

Some women don't even need these signs. They just 'know' that they are pregnant.

Pregnancy tests

Pregnancy tests can be carried out on a sample of urine from the first day of a missed period - that is, about two weeks after conception.

You can collect urine at any time of the day. Use a clean, soap-free, well-rinsed container to collect it. You can get pregnancy tests free or for a small charge from your GP or family planning clinic. Many pharmacists and most pregnancy advisory services also offer tests, usually for a small fee. You can also buy do-it-yourself pregnancy testing kits from pharmacists. They can be expensive but give you a quick result and you can do the test in private. Follow the instructions to be sure of a reliable result.

The results of the test

A positive test result is almost certainly correct. A negative result is less reliable. You could wait a week and try again, or go straight to your doctor.

See your doctor

Whether or not you have had a pregnancy test, you should see your doctor as soon as you think you are pregnant. Being pregnant may affect your GP's treatment of any current or future illness. Your GP will also be able to advise you about antenatal care in your area and put you directly in touch with a midwife if you wish.

Information about the services available is given in the chapters on Deciding where to have your baby and Antenatal care and antenatal classes. It may help to look at these chapters before you talk to your doctor.

Working out when your baby is due

Use the chart below to work out your 'EDD' - expected date of delivery (or 'EDC' - expected date of confinement). Pick out the date of the first day of your last monthly period from the figures in black. The date your baby is due is immediately underneath it in blue.

For example, if your last period started on March 12, your baby will be due around December 17. Remember that this date is no more than a rough guide and it is based on a 28-day cycle. If your cycle is longer or shorter than this, your baby will be due later or earlier than the date shown in blue. In fact only 3% of babies are born on their 'due' date. 90% deliver in the week before or after the EDD.

Knowing that you're pregnant

You may feel very happy or excited when you discover that you are pregnant, but you shouldn't worry if you don't. Even if you have been looking forward to pregnancy, it is not unusual for your feelings to take you by surprise. And if your pregnancy was unplanned, then you may feel quite confused. Give yourself a little time to adjust to the idea of being pregnant. Even though you may feel rather anxious and uncertain now, this does not mean that you won't come to enjoy your pregnancy or to welcome the idea of the baby. Discuss your feelings with your midwife or doctor who will help you to adjust to your pregnancy, or, in England and Wales, will give you advice if you are not happy to continue with it. You may want to share the news with family and friends immediately or wait a while until you've sorted out how you feel. Others in your family/extended family may have mixed feelings. You'll need to talk about these feelings. But do begin to think about your antenatal care (that is, the care you'll receive leading up to the birth of your baby) and where you would like to have your baby. The earlier you begin to organise this, the more chance you will have of getting what you want.