It claims a report based on the calls it received to its legal advice helpline last year provides anecdotal evidence of a "hardening of attitudes" and "discrimination taking place".
It adds that this is "just the tip of the iceberg" and that it hopes the Government will take action to "stamp out" pregnancy and maternity discrimination by employers.
The Working Families' 2012 report showed high levels of maternity discrimination, with the issue raised in 10% of the 2,634 calls received from working parents and carers.
The report also draws attention to the unpicking of flexible working arrangements by employers, with little consideration of the impact on family life. As childcare costs rose and redundancies continued last year, many callers' queries were about leaving the labour market or how they could make work pay.
Working Families has called on the Equality and Human Rights Commission (EHRC) to revisit this issue.
It has also called on employers to recognise the impact that their imposed changes to contracts are having on families already struggling to balance their work and caring responsibilities.
And it has urged the Government to provide help with childcare costs for parents on low incomes, and ensure childcare is available for parents of disabled children.
Working Families chief executive, Sarah Jackson, said: "This is the third year we've reported on high levels of maternity discrimination, with signs that employer attitudes are hardening and discrimination becoming more blatant.
"Eight years ago - before the recession hit - the Equal Opportunities Commission found 30,000 women lost their jobs each year because of pregnancy or maternity.
"It is time the EHRC revisited the issue, as we believe our helpline reveals only the tip of the iceberg."
She added: "We call on the Government to use the forthcoming budget to provide extra help with childcare costs and protect families from further cuts. Our helpline reveals that parents are struggling to make work pay."
