pregnant
The biggest UK study so far on depression in pregnancy found the peak time for experiencing such problems was at 32 weeks, when 13.5 per cent of women scored highly on tests for depression. Smaller studies echo this finding.
It's also clear many women experience stress, anxiety and other forms of emotional distress and mental illness during pregnancy.
Some women are depressed or anxious already, and this may continue or re-emerge when they become pregnant.
Researchers have found if a woman is very anxious during pregnancy, it increases the risk of her baby developing behavioural problems later in life. Heightened maternal anxiety can also affect the stress response of the baby because stress hormones cross the placenta.
Depression during and after pregnancy may increase the risk of the mother having problems bonding with her baby. This doesn't mean if you're experiencing such problems your baby's bound to be affected. The differences are in risk levels - they don't predict anything.
But the possible effect on your baby is one of the reasons your mental health during pregnancy should be taken seriously and treatment offered where necessary.
Pregnancy can be daunting, whether or not you it was planned, and some women have negative thoughts about being a mother.
Having a baby is a massive change in your life and feeling love for someone you've never met is a challenge. But persistently dark thoughts or fears maybe a symptom of depression or anxiety and it will help you if you share them with a midwife or doctor you feel comfortable with.
Your midwife or GP should ask questions about how you're feeling mentally at each antenatal visit, in line with National Institute for Health and Clinical Excellence treatment guidelines.
If your midwife or doctor is following this guidance they should ask two questions, using words similar to these:
During the past month, have you often been bothered by feeling down, depressed or hopeless?
During the past month, have you often been bothered by having little interest or pleasure in doing things?
If you answer 'yes' to either of these questions, you'll be asked if you want to seek help.
You may find you're also asked to complete a questionnaire that aims to assess whether or not you might be depressed.
There's no definitive answer as to whether women with antenatal depression should be given antidepressants. There hasn't been a lot of research into the effects of medication on the health of the unborn baby, or whether it affects labour and birth.
One report, published in 2007, suggests the use of antidepressants is linked to lower birth weights and an increased risk of preterm birth.
You and your doctor will need to weigh the risks and benefits carefully, bearing in mind your individual needs and background.
NICE recommends women who are mildly or moderately depressed in pregnancy should be offered self-help or talking therapies, including counselling. If the depression becomes worse or doesn't respond to such treatments, antidepressants can be considered.
Serious mental illness may require further intervention.
Talk to your doctor about your choice of treatment. If you and your doctor decide you need medication, some drugs have been shown to be safer for use in pregnancy than others.