Most pregnant women only experience the minor complications of pregnancy, but for a minority more serious problems occur.
In extreme cases, these can result in the death of the baby and, very rarely in developed countries, that of the mother.
Problems range from failure of the embryo to implant in the womb, leading to miscarriage, to rupture of the placenta and pre-eclampsia.
The foetus can also develop problems in the womb, including genetic defects.
Blood tests are offered in the early stages to screen for Down's syndrome, spina bifida and other chromosomal abnormalities.
The tests - for alpha-fetoprotein levels - cannot predict if the baby will have an abnormality.
They only predict probability. If probability is high, the woman will be offered further more accurate tests, including amniocentisis.
This involves sticking a needle into the womb to take a sample of amniotic fluid. It carries a small risk of miscarriage.
Ultrasound scans can also detect abnormalities as well as slow-growing foetuses, which may be the result of poor blood supply or other complications.
To minimise the possibility of the foetus developing spina bifida, woman are advised to take folic acid supplements in the months leading up to conception and the first three months of pregnancy.
Miscarriage - when the pregnancy is terminated before the sixth month - is estimated to occur in between 10 to 20% of pregnancies, but this may be an underestimate as many women may miscarry before they are even aware they are pregnant.
Miscarriage usually happens in the first three months.
Early miscarriage is mainly due to the foetus failing to develop normally.
Later miscarriage is more likely to be the result of the placenta not functioning properly or a weak cervix.
Symptoms include bleeding, but this is not always the case and about half of all women who bleed in the early stages of pregnancy do not go on to miscarry.
Some women continue to have a brief period during their pregnancy and bleeding may also not be related to the foetus at all but could be caused by lesions in the vagina or cervix.
Other implantation problems include ectopic pregnancy where the fertilised egg becomes embedded outside the womb.
Ectopic pregnancy is now the most common cause of maternal death in the first three months of pregnancy in the west and it has been increasing steadily in the UK in recent years.
This is probably because other causes of maternal death have decreased due to better care.
However, 90% of cases are now diagnosed in the early stages before life-threatening complications arise.
The reasons for ectopic pregnancy are unclear, but research suggests damage to fallopian tubes caused by infections, such as sexually transmitted diseases, could be a factor.
Symptoms include severe pain and bleeding. The woman may not know she is pregnant.
Because the condition is being diagnosed earlier due to more advanced technology, it often does not cause lasting damage and the woman can go on to have a normal pregnancy afterwards.
Pre-eclampsia - or pregnancy-induced hypertension - is thought to occur in 10% of pregnancies and is the commonest cause of maternal death in the UK, killing around 10 women a year.
It also leads to the death of some 1,000 babies a year.
Symptoms include high blood pressure, fluid retention and protein in the urine.
Blood pressure and urine are now regularly checked throughout pregnancy.
Pre-eclampsia usually occurs towards the end of the pregnancy and the effects can be felt for some days after the birth.
Some women are thought to have a genetic predisposition to it.
Women are more likely to suffer from the condition in their first pregnancy and most do not go on to experience it again in subsequent pregnancies.
The causes of pre-eclampsia are unclear, but research suggests it may be linked to an immune reaction to the foetus or the placenta.
If the condition is serious, women may be advised to rest or take drugs to lower their blood pressure and, in some cases, an early caesarean or induction may be performed.
Placenta praevia and placenta abruptio:
Both these conditions result in bleeding in the later stages of pregnancy and can be life-threatening for the baby, but are relatively rare.
Many pregnant women are told they have a low-lying placenta.
In most cases this moves upwards during the course of the pregnancy.
However, in a small proportion of cases, the placenta remains near the opening of the cervix and can lead to bleeding and potentially to the loss of the baby.
This is known as placenta praevia. A caesarean is usually necessary and, in severe cases, women may require a blood transfusion.
With placenta abruptio, the placenta becomes detached from the lining of the womb, resulting in severe pain and bleeding which may flow down and out through the vagina or upwards and in the foetus being starved of its oxygen supply.
With both conditions, the causes are unclear and recurrence in a subsequent pregnancy is unlikely.
Women who bleed in later pregnancy are advised to contact their hospital immediately.