The results also show that a Western diet – high in potatoes, meat, white bread and margarine – increase the odds of developing these conditions during pregnancy, they said.
“Generally, it is safe to consume fish during pregnancy”
They said their findings should encourage women at risk of developing gestational hypertension and planning to conceive, or already pregnant, to eat a well-balanced healthy diet to reduce their risk.
Hypertension affects around one in 10 pregnancies, noted the authors of the large study published in BJOG: An International Journal of Obstetrics and Gynaecology.
The most common forms being pre-existing high blood pressure that develops during pregnancy – gestational hypertension – and pre-eclampsia which develops from around 20 weeks of gestation.
The new study, which involved 55,138 women who took part in the Danish National Birth Cohort, was carried out by a team of UK and Scandinavian researchers.
They took part in two phone interviews during pregnancy, at 12 and 30 weeks of gestation, and in two at six and 18 months after birth. A questionnaire assessed diet at 25 weeks of gestation.
“It is also vital that women and their partners are encouraged to manage their weight”
The results found a diet rich in vegetables and fish decreased the odds of developing gestational hypertension by 14% and pre-eclampsia by 21%.
Meanwhile, a Western diet increased the odds of developing gestational hypertension by 18% and pre-eclampsia by 40%.
In total, 14% of women had gestational hypertension, 2% had pre-eclampsia and 0.4% had severe pre-eclampsia.
On average, women with high blood pressure or pre-eclampsia had a higher body mass index (1.6-2.3kg/m2 higher) than those without the conditions.
While the research only shows an association between diet and hypertension risk in pregnancy, the results add to previous evidence supporting a healthy, well-balanced diet, said the study authors.
Lead author Emmanuella Ikem, from the University of Bristol and Imperial College London, said: “Our findings support the importance of eating a healthy and well-balanced diet in vegetables and fish and cutting out processed foods where possible.
“This will help to reduce a woman’s risk of developing high blood pressure and pre-eclampsia during pregnancy,” she said.
“Current advice recommends eating at least five portions of different fruit and vegetables every day, instead of foods high in fat,” noted Ms Ikem.
“And, generally, it is safe to consume fish during pregnancy – no more than two portions of oily fish, such as mackerel or salmon, a week, and no more than two fresh tuna steaks or four medium-sized cans of tuna a week. Women should avoid eating shark, swordfish or marlin,” she added.
Dr Pat O’Brien, a spokesman for the Royal College of Obstetricians and Gynaecologists, said: “High blood pressure and pre-eclampsia can result in harmful complications for mother and baby.
“These latest findings are encouraging as it shows there are additional steps a woman can take to reduce her risk of these conditions by eating healthily,” he said.
He added: “It is also vital that women and their partners are encouraged to manage their weight and to have a healthy diet ideally before conception, to ensure the healthiest possible pregnancy and best start to their child’s life.”