Pregnant women are being made to feel they have let their company down, a new survey has shown.
The study from baby charity Tommy's and People Management Magazine also found that one in five human resources (HR) professionals has been approached by a pregnant employee who says she has been discriminated against.
Over 600 HR professionals were questioned from a range of industries by the two organisations.
During pregnancy women need some time off for medical reasons to ensure the health of both mother and baby.
But the survey found that one in six HR workers have seen a pregnant employee being put under pressure to over perform to make up for lost time due to extra time off for appointments such as check-ups.
One company even encouraged new mothers to return to work earlier than the statutory maternity leave by offering a financial bonus.
Nearly one fifth of those questioned said they had witnessed pregnant employees being passed over for promotion, training and development.
One in five companies were found to not have updated their maternity policy despite extensive changes in the law earlier this year.
Tommy's chief executive Jane Brewin described the survey's findings as "alarming".
"It is clear that pregnant women in the workplace are continuing to be made to work under pressure and to be made to feel guilty from the moment they have announced their pregnancy," she said.
"Not only are they being dropped from key meetings and promotion but they are sometimes not even able to return to their original position of employment."
Commenting on the findings, Jenny Watson, chair of the Equal Opportunities Commission (EOC) said: "It is appalling that women continue to be passed over for promotion, mistreated and put under stress during pregnancy.
"This discrimination - fuelled by a lack of understanding of how to manage pregnancy at work - drives thousands of women out of the workplace and causes misery to the parents who suffer at this important time. But in good-practice workplaces, more than nine in ten pregnant women return to work.
"If more employers are to follow this trend, and avoid squandering a vast pool of talent, we need to see more HR and financial support, particularly for small and micro businesses."