A review of 30 clinical studies across nine supplement types — with clear answers on what helps, what does not, and who benefits most.
Premature birth — delivery before 37 completed weeks — affects roughly one in ten pregnancies worldwide. For the families it touches, it brings weeks in neonatal intensive care, uncertainty about long-term development, and a difficult start to life for a baby who arrived before they were ready.
It is natural that expectant mothers and their healthcare providers want to do everything possible. Supplements are an obvious question: they are accessible, widely available, and frequently discussed in pregnancy communities. But does the evidence support their use for preventing premature birth?
We reviewed 30 peer-reviewed clinical studies covering nine supplement types — ranging from omega-3 fish oil to vitamin C — to find out what the research genuinely shows. The findings are more nuanced than either enthusiastic supplement advocates or blanket sceptics might suggest.
The Honest Overview: No Single Supplement "Prevents" Premature Birth
Before diving into the detail, the most important thing to understand is this: no supplement has been shown to prevent premature birth across all pregnant women. This is not a failure of the research — it reflects the reality that premature birth has many causes, including infection, cervical insufficiency, placental problems, and metabolic conditions. A supplement that addresses one pathway cannot be expected to solve all of them.
What the research does show is that certain supplements may reduce risk in specific groups of women — particularly those with identifiable nutritional deficiencies or metabolic risk factors. The evidence for omega-3 fish oil and vitamin D is the most developed, though both come with important caveats. Several other supplements that are widely taken in pregnancy — including vitamin C — show clear, high-certainty evidence of no benefit for this specific outcome.
Understanding which group you fall into matters more than any universal recommendation.
Omega-3 Fish Oil: Promising for Some, Not All
Of all the supplements reviewed, omega-3 fatty acids (DHA and EPA, found in fish oil) have attracted the most research attention for premature birth prevention. The picture that has emerged is genuinely interesting — and more complex than many summaries suggest.
The most important study in this area is the ORIP trial, published in the New England Journal of Medicine in 2019. Researchers randomised 5,544 pregnant women to receive either fish oil (900 mg/day of DHA and EPA combined) or a control, and tracked rates of early preterm delivery.
The headline result was neutral: fish oil did not reduce premature birth risk in the overall group. But a pre-planned subgroup analysis told a more interesting story. Women who started the trial with low blood omega-3 levels showed a meaningful reduction in early preterm delivery. Women who started with high omega-3 levels saw no benefit — and there was a suggestion of slightly increased risk at very high baseline status.
This finding is biologically plausible. Omega-3 fatty acids help dampen inflammatory signals in the body, and inflammatory pathways are known to be involved in triggering early labour. If you already have adequate omega-3 levels, supplementing further may not add anything useful. If your levels are genuinely low — common in women who eat little or no oily fish — topping up may matter.
A broader meta-analysis of 26 randomised trials found a modest average reduction in preterm birth risk with omega-3 supplementation (around 8%), but the quality of evidence was graded as low and the results were mixed across trials. The key conditional finding — that benefit concentrates in women with low baseline omega-3 status — has not yet been tested in a large, prospectively designed trial that enrolled specifically low-status women. That study is still needed.
Practical implication: If you eat oily fish regularly (salmon, mackerel, sardines two or more times per week), the evidence does not suggest omega-3 supplements will reduce your premature birth risk further. If you rarely eat oily fish, discussing omega-3 supplementation with your midwife or doctor — particularly if you are already at elevated risk of premature birth — is reasonable.
Vitamin D: The Strongest Signal, Especially in Gestational Diabetes
Vitamin D has arguably the most compelling evidence of any supplement in this area, though it is important to understand where that evidence comes from.
Two independent meta-analyses — published in 2021 (Wang et al., covering 17 trials, 1,432 women) and 2023 (Wu et al., covering 20 trials, 1,682 women) — both found that vitamin D supplementation was associated with a substantially lower rate of premature birth in women with gestational diabetes mellitus (GDM). The effect sizes were striking: roughly a halving of premature birth risk in the GDM group across both analyses.
The reason this appears to work is likely indirect. Gestational diabetes — high blood sugar developing during pregnancy — is itself a significant risk factor for premature delivery, partly because poorly controlled GDM can lead to complications requiring early induction of labour. Vitamin D plays a role in insulin sensitivity and blood sugar regulation. By improving glucose metabolism, vitamin D may reduce GDM-related complications, and with them, the downstream risk of premature delivery.
A smaller, earlier meta-analysis also found a supportive signal for vitamin D in general pregnancy populations, though this evidence is lower certainty.
The main caveat here is that most of the trial data comes from Chinese and lower-income country settings, where baseline vitamin D deficiency and dietary patterns differ from those typical in the UK and Europe. Whether the same effect size would be seen in a British pregnancy population is genuinely unknown. It is also worth noting that no standardised vitamin D dose for pregnancy has been established by this research — the studies used different doses and forms.
What is clear is that vitamin D deficiency during pregnancy is associated with multiple adverse outcomes, and correcting deficiency is supported on established bone health and immune function grounds regardless of the premature birth question. UK public health guidance already recommends a 10 mcg (400 IU) supplement for all pregnant women during autumn and winter.
Myo-Inositol: Emerging Evidence in a Specific Group
Myo-inositol is a naturally occurring compound related to the B-vitamin family, found in wholegrains, fruits, and legumes. In recent years it has attracted attention as a supplement that may help regulate blood sugar and reduce GDM risk.
A 2022 meta-analysis of eight randomised trials found that myo-inositol at 4 g/day was associated with a significantly lower rate of premature birth in women at elevated GDM risk (OR 0.41 — roughly a 60% reduction in odds). The mechanism is similar to vitamin D: by reducing the likelihood of developing GDM, myo-inositol may secondarily reduce the premature birth complications that GDM brings.
There are important limits to this evidence. It applies only to women with identifiable GDM risk factors — women with obesity, a previous GDM diagnosis, a family history of type 2 diabetes, or polycystic ovary syndrome. It has not been tested in the general pregnant population. There is also an important dose note: the benefit appears specific to myo-inositol alone at 4 g/day — a combined myo-inositol and D-chiro-inositol formula did not show the same effect in trials.
This is an area of active research, and the GDM-prevention evidence for myo-inositol is growing. For women with GDM risk factors, it is worth discussing with a healthcare provider.
Vitamin C: A Clear Null Result
Not all supplements are a matter of nuance and subgroups. For vitamin C, the evidence is unusually clear.
A 2025 systematic review and meta-analysis pooled data from 17 randomised controlled trials covering more than 21,500 women. The finding: vitamin C supplementation (typically 500–1,000 mg/day, often combined with vitamin E) had no effect whatsoever on premature birth risk. The summary risk ratio was 1.04 — essentially 1.0 — with a tight confidence interval ruling out any meaningful effect in either direction.
This is a high-certainty null result. Secondary outcomes — gestational age at delivery, rates of very early preterm birth — were also unaffected. Vitamin C is widely taken in pregnancy for its antioxidant properties and immune support benefits, and it is a safe supplement at the studied doses. But for premature birth prevention specifically, current evidence is clear that it does not help.
What About Calcium, Zinc, Probiotics, and Folate?
The picture for these supplements is similar: insufficient or absent evidence of a premature birth benefit.
Calcium: The most recent Cochrane review (2025, 37,504 women across 10 trials) found very uncertain evidence for any calcium-related reduction in premature birth. A separate 2024 meta-analysis appeared to show a strong pre-eclampsia benefit from calcium, but a trustworthiness analysis excluded a large number of its included trials, substantially undermining that finding.
Zinc: Two well-designed studies — an RCT of 540 women and a Cochrane review of seven trials — found no premature birth benefit from zinc supplementation over standard care.
Probiotics: No significant benefit was seen for premature birth prevention across any study type reviewed, including trials in GDM populations, general pregnancy, and vaginal microbiome modification.
Folate and multiple micronutrients: Essential for neural tube defect prevention — a well-established, unrelated benefit — but a large cohort study of more than 16,000 women found no association between periconceptional folate supplementation and premature birth risk.
The Key Takeaway: Know Your Risk Profile
The message from this research is not "supplements do not work" — it is that supplement benefit for premature birth is population-specific and evidence-dependent.
If you have gestational diabetes or significant GDM risk factors, the evidence for vitamin D and myo-inositol is meaningful and worth discussing with your healthcare team. If you eat little or no oily fish, omega-3 supplementation before 20 weeks of pregnancy is a reasonable conversation to have with your midwife. For everyone else, the evidence base does not currently support additional supplementation specifically for premature birth prevention beyond routine pregnancy guidance.
And for vitamin C, zinc, and calcium taken specifically in hopes of preventing premature birth — the research says clearly that these supplements do not reduce that risk.
The most important step is not choosing the right supplement — it is having a conversation with your midwife or obstetrician about your specific risk factors. Premature birth has many causes, and a personalised approach grounded in your own health history will always be more useful than a general supplement recommendation.
📄 Explore the Full Research
Want to go deeper? Our research team has reviewed 30 clinical studies in full.
- Clinical Evidence One-Pager (PDF) — concise evidence summary for clinicians and healthcare providers
- Informative One-Pager (PDF) — plain-language summary for expectant mothers and families
- Full Research Paper (PDF) — complete literature synthesis with evidence tables and 29 citations
This article is based on a structured review of 30 peer-reviewed clinical studies (evidence current as of March 2026). It is for informational purposes only and does not constitute medical advice. Food supplements should not replace a varied diet and healthy lifestyle. Always consult a healthcare professional before starting any supplement, especially during pregnancy.