Almost a third of pregnant women suffer strong feelings of depression and anxiety, according to a new survey.
The poll found that many are struggling with signs of antenatal depression - depression in pregnancy - but are afraid to tell their doctor or midwife.
NHS data suggests up to 15% of women suffer antenatal depression, although the new survey suggests this could be far higher.
Some 30% of mothers-to-be frequently experience five or more key indicators of antenatal depression, according to the poll.
These include feeling anxious for no reason, losing interest in day-to-day activities and feeling so unhappy they cry.
If the figure was extrapolated across all pregnancies, this suggests almost a quarter of a million women a year in the UK suffer from depression during pregnancy.
The poll of 1,000 mothers and pregnant women, from BabyCentre, also found that 42% had never told their doctor or midwife about their symptoms of depression.
The top three reasons were that they felt guilty, embarrassed or worried that others would judge them.
Almost half did not want to be labelled as mentally ill, while 26% had not even discussed the issue with a partner, close friend or relative.
More than one in five (21%) had not done anything at all to address their symptoms.
Sasha Miller, international managing editor of BabyCentre, said: "Our study paints a stark picture of the alternative face of pregnancy - it's not all baby showers, blossoming bumps and baby moons.
"While pregnancy is an emotional time for any woman and occasional mood swings are normal, so many women experiencing so many symptoms so much of the time is a serious problem.
"Women feel under pressure to act like they are having a perfect pregnancy but the reality is very different for huge numbers of mums-to-be.
"There is still a stigma attached to depression and our research shows that admitting to suffering from symptoms whilst pregnant is something many expectant mums feel unable to do. As a result they aren't seeking the help and support they need from health professionals. This needs to change."
When asked about their biggest worries regarding life with a newborn, developing postnatal depression was mentioned by more women who felt depressed in pregnancy than any other concern.
This came ahead of not having enough money or their baby's health.
Dr Patrick O'Brien, spokesman for the Royal College of Obstetricians and Gynaecologists (RCOG), said: "We know that mental health problems during pregnancy and following delivery are common.
"It is therefore vital that women receive the right support and at the right time.
"Every obstetric unit should have in place a clearly defined care pathway for referring women to local specialised perinatal mental health services.
"More importantly, we need to identify women who are at-risk of mental illness and ante or postnatal depression so that we can support them and ensure that they receive the best possible care."