Pregnancy occurs after fertilisation, when a man's sperm penetrates a woman's egg.
If you suspect that you are pregnant or have taken a home pregnancy test that confirms you are pregnant, arrange to see your GP or a midwife straight away.
Development of the egg
Five to seven days after ovulation (the release of an egg from the ovary), the fertilised egg implants itself into the wall of the womb and produces root-like outgrowths called villi. These help to anchor it to the lining of the womb and will eventually grow into the placenta (the organ that feeds and protects the baby until birth).
The placenta maintains and nourishes the baby by enabling the transfer of oxygen, amino acids, fats, vitamins and minerals from the mother's blood. It also allows the transfer of waste substances from the baby.
Embryonic and foetal stages
From the time of implantation into the wall of the womb until approximately the eighth week of life, the developing baby is known as an embryo. Development is rapid during this stage, as specialised cells begin to form the vital organs, nervous system, bones, muscles and blood.
After the eighth week of pregnancy, the developing baby is called a foetus. It is about 2.4cm (1 inch) long, with most of the internal organs formed. External features, such as the eyes, nose, mouth and ears, can be seen and fingers and toes start to appear.
As the unborn baby grows, so does the womb. A fluid-filled double membrane surrounds the baby. This normally breaks when the baby is ready to be born, releasing the amniotic fluid (the liquid that surrounds the baby).
During pregnancy, the baby floats freely in the amniotic fluid and constantly swallows this fluid, excreting it in their urine. Much information about the baby's health can be obtained during a procedure called amniocentesis, where a small sample of amniotic fluid is taken for testing. However, amniocentesis is quite invasive (involves going into the body) and carries a small risk of miscarriage, so it is usually only offered to women when there is a significant risk that their baby will develop a serious condition or abnormality.
The care you receive from your GP, midwife and other health professionals during your pregnancy is known as antenatal care. The purpose of antenatal care is to track the progress of your baby and to monitor both your and the baby's health.
If this is your first baby, you will probably be offered 10 antenatal appointments. If you have had children before, you will probably be offered seven appointments.
For more information, see Health A-Z: antenatal health and care.
Length of pregnancy
The length of a normal pregnancy varies between about 37 and 42 weeks, although the delivery date is calculated at 40 weeks from the first day of your last period. Only around 5% of babies are born on their due date. Babies born before 37 weeks are considered premature.
Possible complications that can occur during pregnancy include:
miscarriage: loss of a pregnancy during the first 23 weeks
ectopic pregnancy: when a fertilised egg is implanted outside the womb
pre-eclampsia: a problem with the placenta that causes high blood pressure in the mother and growth problems in the baby
stillbirth: a baby born after the 24th week of pregnancy who does not show any signs of life
More unusual complications are described below.
Placenta praevia and vasa praevia
Placenta praevia occurs when the placenta covers part or all of the cervix (the baby's exit from the womb). It may cause bleeding in the pregnancy and, if the placenta does not move up as the pregnancy progresses, a caesarean birth will be necessary. In most women, the placenta moves out of the way before birth.
Where an earlier placenta praevia has moved out of the way before birth, the risk of a vasa praevia must be eliminated. A vasa praevia occurs when one or more of the baby's placental or umbilical blood vessels cross the entrance to the cervix beneath the baby, and can be ruptured or compressed when labour starts. If this is diagnosed, a caesarean birth will be necessary.
Obstetric cholestasis is a rare complication that occurs during pregnancy and is caused by a build-up of bile (a naturally occurring chemical) in your bloodstream. The main symptom is intensely itchy skin.
The exact cause of obstetric cholestasis is not known, although some researchers suggest that it may be a result of a reaction to the hormone changes that occur in your body during pregnancy. The condition may also run in families.
Obstetric cholestasis can cause serious complications, significantly increasing the risk of your baby being born prematurely.
An estimated one in five women experiences some itching during their pregnancy. However, for most women it is a result of the stretching of their skin, as well as increased blood supply to the skin. If your itching becomes particularly intense, contact your GP or midwife for advice.
If you are diagnosed with obstetric cholestasis, you and your baby will be carefully monitored. Medication can help control the itching and you may need an induced labour or caesarean birth to ensure that your baby is not affected by the build-up of bile.
A molar pregnancy is when the fertilised egg cannot develop into an embryo due to genetic abnormalities. However, the layer of cells that would eventually become the placenta continues to develop and produce the pregnancy hormone hCG (human chorionic gonadotrophin) at an abnormally high level.
Most molar pregnancies are diagnosed early, perhaps as a result of bleeding or following an ultrasound scan. If left untreated, molar pregnancies normally result in miscarriage.
You will usually need a small operation, carried out under general anaesthetic. After treatment, your level of hCG will probably be monitored. In most people, the level will fall. If it stays the same or continues to rise, further treatment may be needed.
After a molar pregnancy, you may be advised to wait for 12 months before trying for another baby.