The Royal College of Midwives (RCM) said it was concerned by reports that two maternity units could close.
Norfolk has maternity units at hospitals in Norwich, Gorleston and King's Lynn, with the latter two also serving Cambridgeshire and Suffolk.
The trade union posted on Facebook, external that local maternity services should be invested in rather than cut.
The Norfolk and Waveney University Hospitals Group (NWUHG) has set out its "Case for Change", external on how services could be delivered in future, but stressed that no decisions had been made and no consultation had begun. NHS leaders said any move to concentrate services at one or two sites would be based on providing safer care for women and babies.
The BBC also understands the health secretary is talking to the chief executive of NHS England regarding what has been happening in Norfolk.
The RCM warned that closing units serving rural and isolated communities could mean women travelling "unacceptable distances" while in labour.
If maternity services were removed, the Norfolk and Waveney University Hospitals Group (NWUHG) said that neonatal units and children's inpatient wards would also close because the services were inter-dependent.
Options include keeping services as they are, or closing consultant-led maternity units at the James Paget Hospital in Gorleston, the Queen Elizabeth Hospital in King's Lynn, or both.
As well as women in Norfolk, the plans could affect women in Lowestoft and north Suffolk who give birth in Gorleston and those in Wisbech and north-east Cambridgeshire who travel to King's Lynn.
The RCM argued the proposals risked worsening existing inequalities in maternity outcomes and said the answer was investment and more staffing, rather than allowing services to "shrink".
It also said closures could result in midwives being redeployed further from home.
The Facebook post said: "Women and babies deserve safe, local care, and closing units is not the answer."
A Department of Health and Social Care (DOHSC) spokesperson said:
"While decisions about individual NHS maternity services are made locally by NHS trusts, any changes to services must be based on clear evidence that they will deliver better outcomes for women and babies."
It added that: "We expect that any further development of this work will involve patients and the public and carefully consider issues such as access, travel times and the needs of local communities."
The department said any future plans would need to take account of National Maternity Standards, external and the upcoming Maternity and Neonatal Action Plan., external
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Two hospitals at risk of losing maternity units
NWUHG said 60 clinicians had helped develop the review.
The Case for Change argues that falling birth rates, workforce shortages and increasing numbers of women requiring more complex care are making it harder to sustain specialist services at every site.
Lesley Dwyer, NWUHG chief executive and a former midwife, previously told the BBC the review was one of the most significant pieces of work undertaken by the organisation and was aimed at making services sustainable in the long term.
"The scenarios examined so far are a starting point for further assessment, not formal proposals," she said.
"No preferred approach has been identified and no decision has been made to close or relocate these services.
"Patient safety must remain at the centre of this work. We also need to understand what different approaches could mean for choice, travel, access and the experiences of women, babies, children and families."
The work comes after NHS bosses said that smaller maternity units across the country may have to close.
The Case for Change will now be submitted to NHS England before further research and any public consultation takes place.
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Norfolk and Waveney case for change documents
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