Researchers discovered that most of these women lacked essential elements typically found in meat and dairy products, such as vitamins B12, B6, D, folic acid and riboflavin.
These are crucial for foetal development in the womb.
Prof Keith Godfrey, the lead author and an expert in epidemiology at the University of Southampton, told The National: "It is well-known those following vegan diets do not obtain sufficient quantities of several vitamins – for example, they are recommended by the NHS to have fortified foods or supplements containing vitamin B12 and vitamin D.
"Previous reports have summarised the world literature, showing that veganism is associated with low intake of vitamins B2, niacin [B3], B12, D, iodine, zinc, calcium, potassium and selenium, and likewise for vegetarians."
The study, conducted by a team of international scientists led by the University of Southampton and published in PLOS Medicine, has brought to light the trend affecting pregnant women in high-income countries, including the UK, New Zealand and Singapore.
It challenges the common perception that nutrient deficiencies are primarily a concern in lower-income nations.
The study highlights that even in wealthier countries, expectant mothers are not immune to such deficiencies, which can have profound implications on the health and development of their infants.
The research involved a survey of more than 1,700 women, with a focus on those trying to conceive and those already pregnant.
"Our findings cover both omnivorous and vegetarian/vegan women, indicating that more than nine in 10 of whom have low or marginal status for key vitamins," Prof Godfrey said.
Prof Godfrey expressed serious concern about the prevalence of vitamin deficiencies among women in wealthy countries.
He highlighted the potential exacerbation of this issue as diets shift towards plant-based alternatives to meet net-zero carbon emission goals.
"The push to reduce our dependence on meat and dairy is likely to further deplete expecting mothers of vital nutrients," he said.
"With the need for sustainable, plant-based diets, it is likely that higher levels of supplement use will be required. Longer-term plans require agricultural production of plant crops with higher vitamin content," Prof Godfrey told The National.
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Prof Wayne Cutfield from the University of Auckland in New Zealand also emphasised the importance of maternal well-being before and during pregnancy.
He suggested over-the-counter multivitamins could be a viable solution to reduce deficiencies.
"The well-being of a mother ahead of conceiving and during a pregnancy has a direct influence on the health of the infant," he said.
Associate Prof Shiao-Yng Chan from the National University of Singapore warned of the increasing vitamin deficiencies if the trend towards less meat and dairy consumption continues.
She advocated women taking more supplements or receiving specific dietary advice to ensure sufficient intake of essential micronutrients.
This study is the first to demonstrate over-the-counter supplements can effectively reduce vitamin insufficiencies during preconception, pregnancy and lactation.