Hydration in Pregnancy and Breastfeeding: What Actually Changes

Your blood plasma volume increases by nearly half by late pregnancy. That single fact explains most of what changes about fluid. And it also explains why two of the most repeated pieces of pregnancy hydration advice are wrong.

Most pregnancy hydration advice is some version of "drink more, you're drinking for two."

That's not wrong, but it's uselessly vague. And it sits alongside two specific pieces of advice that circulate constantly and don't survive contact with the evidence: that you should cut salt to reduce swelling, and that magnesium supplements meaningfully lower preeclampsia risk.

This guide starts with the actual physiology, which is more interesting than the slogan, then addresses both claims directly.

Before you read on: this guide covers situations that can become medical emergencies. Read the scope and safety note first.

The Short Answer

Bottom line: your plasma volume expands by nearly half by late pregnancy and that expansion has to be filled, which is the real reason fluid needs rise. But two common pieces of advice don't hold up: sodium restriction for swelling isn't supported by current intake guidance, and magnesium supplementation for preeclampsia isn't supported by the Cochrane evidence.

Three things worth carrying:

• The physiology is dramatic and normal. Plasma volume rises roughly 48% (about 1,150 mL) by weeks 35 to 38.
• Sodium guidance doesn't change in pregnancy. The adequate intake stays at 1,500 mg/day, the same as for non-pregnant adults.
• Magnesium sulfate and magnesium supplements are different things. One is a monitored IV treatment; the other is a pill. Evidence for the first says nothing about the second.

What Actually Changes in Your Body

Your blood plasma volume expands enormously. And progressively. This is the mechanism behind almost everything else on this page.

A systematic review of plasma volume in pregnancy pooled measurements against non-pregnant baseline:

At peak that's 48%, or about 1,150 mL, above the non-pregnant volume.

Two things follow. First, the steepest climb is in the first half of the second trimester. The review found the rate of gain roughly tripled between the first trimester and early second (148 vs 450 mL). So the period when fluid demand is rising fastest is often the period when people feel most normal and least focused on it.

Second, and more importantly: this expansion is supposed to happen. It supports blood flow to the placenta and provides reserve against blood loss at delivery. It isn't fluid retention to be minimised. That framing matters for the salt section below.

How Much Fluid Do You Actually Need?

The National Academies set total water intake in pregnancy at 3.0 litres per day, and "total water" is the part people misread.

That figure comes from the Dietary Reference Intakes for water and electrolytes, and it means water from all sources: plain water, other drinks, and the water contained in food. Food typically contributes around a fifth of total water intake, so the plain-water portion is meaningfully less than three litres.

Practical translation, with the usual caveat that your clinician's number beats a population reference:

• Count everything. Milk, soup, fruit, decaf tea, yoghurt all contribute.
• Spread it out. Large volumes at once are harder to tolerate against a compressed stomach later in pregnancy, and worse for nausea earlier on.
• Adjust upward for heat and activity, as you would outside pregnancy. Our hydration calculator covers the general adjustment logic, though a pregnancy target should be confirmed with your provider.

If nausea is the barrier rather than forgetting, the tactics that help are mechanical rather than motivational: small volumes, cold fluids, ice chips, and counting food moisture. We cover that approach in more detail in the GLP-1 guide's nausea section, where the barrier is similar even though the cause differs.

Should You Cut Salt for Swelling?

Current intake guidance doesn't support routine sodium restriction in pregnancy. And the advice to cut salt for swelling is one of the more persistent pieces of outdated folklore.

The National Academies set the sodium adequate intake at 1,500 mg per day for ages 14 to 50, and reaffirmed that value in their 2019 review. That's the same figure for pregnant and non-pregnant people in that age range. Pregnancy does not come with a lower sodium target.

The reasoning connects back to the plasma volume data above. Expanding blood volume by nearly half requires sodium. It's the osmotic anchor that holds that volume in the vascular space. Restricting sodium to reduce visible swelling works against a process the pregnancy needs.

Mild swelling of the feet and ankles in later pregnancy is common and generally benign. Sudden or severe swelling (particularly of the face and hands, or with headache, visual changes or upper abdominal pain) is a different matter entirely and needs same-day medical assessment, because it can signal preeclampsia. That is not a hydration problem and not something to manage with dietary tweaks.

The distinction to hold onto: gradual mild swelling is usually normal physiology. Sudden swelling is a red flag. Neither is treated by cutting salt on your own initiative.

If a clinician has specifically told you to restrict sodium (for chronic hypertension or a kidney condition, say) that instruction is individualised and takes precedence over a population reference value.

Does Magnesium Prevent Preeclampsia?

The Cochrane review says the evidence isn't there. And there's a second, more important confusion underneath this claim.

What the evidence review found

The Cochrane review of magnesium supplementation in pregnancy concluded there is not enough high-quality evidence to show that dietary magnesium supplementation during pregnancy is beneficial.

The detail that matters: when the analysis was restricted to high-quality trials, none of the review's primary outcomes (perinatal mortality, small-for-gestational-age, or preeclampsia) differed significantly between supplemented and control groups. The apparent benefits came from weaker studies and didn't survive the quality filter.

You'll see figures circulating claiming magnesium reduces preeclampsia risk by around 30%. Those come from smaller, more favourable analyses rather than from Cochrane's, and they're generally quoted without that context.

The confusion that actually matters

Here's the more consequential error: magnesium sulfate and oral magnesium supplements are not the same thing.

• Magnesium sulfate is an intravenous medication administered in hospital, under monitoring, to treat preeclampsia and eclampsia and to prevent seizures. Its evidence base is strong and well established.
• An oral magnesium supplement is a different compound, at a different dose, by a different route, taken unmonitored.

Evidence for the first tells you nothing about the second. When you see "magnesium is proven to help with preeclampsia," it's usually the hospital treatment's evidence being borrowed to sell the pill.

Magnesium is also one of the supplements where kidney function genuinely matters for safety. In pregnancy specifically, this is a conversation with your obstetric provider rather than a self-directed purchase.

No product recommendation appears anywhere on this page. None of the electrolyte products we cover elsewhere on this site are formulated or tested for pregnancy, and recommending a supplement to a pregnant reader on the strength of general-population evidence isn't something we're willing to do.

What Changes When Breastfeeding

Fluid needs rise again, for a straightforward reason: you're producing a substantial volume of a fluid every day.

Breast milk is mostly water, and producing it draws on your own fluid balance. The dietary reference intakes set a higher total water intake for lactation than for pregnancy, for exactly this reason.

The practical pattern most people land on:

• Drink at feeds. Thirst often arrives sharply during letdown; keeping water within reach of wherever you feed handles most of it without any tracking.
• Use thirst, which works better here. Unlike some situations we cover on this site where thirst lags badly, the thirst response during lactation tends to be prompt and reliable.
• Watch for the dip when feeds get less frequent. Fluid intake habits often loosen as feeding spaces out, even while production continues.

One thing worth saying plainly, because it's marketed heavily: drinking extra fluid beyond satisfying thirst has not been shown to increase milk supply. Adequate hydration supports production; surplus hydration doesn't boost it.

When It's More Than Normal

Some of these are hydration problems. Several aren't, and the difference is worth knowing in advance.

Seek prompt medical care for:

• Persistent vomiting where you can't keep fluids down. This can indicate hyperemesis gravidarum, which is not ordinary morning sickness and often needs medical treatment, sometimes including intravenous fluids. It's not something to manage with sips at home.
• Sudden or severe swelling, especially face and hands, or with headache, visual changes, or upper abdominal pain: possible preeclampsia.
• Markedly reduced urination, dizziness on standing, confusion, or a racing heart at rest.
• Reduced fetal movement, which always warrants contact regardless of hydration.
• Fever with vomiting or diarrhoea, where fluid losses compound quickly.

For general dehydration warning signs outside the pregnancy-specific ones, our guide to the signs that don't involve thirst covers the broader picture, though in pregnancy, err toward contacting your provider sooner rather than working through a checklist.

FAQ

How much water should you drink during pregnancy? The National Academies set total water intake during pregnancy at 3.0 litres per day. That figure includes water from food and all beverages, not just plain water. Roughly a fifth of most people's total water intake comes from food. Your own clinician's advice takes precedence, particularly with any kidney, heart or blood-pressure condition.

Should you cut salt during pregnancy to reduce swelling? Current dietary reference intakes don't support routine sodium restriction in pregnancy. The National Academies set the sodium adequate intake for ages 14 to 50 at 1,500 mg per day and reaffirmed it in their 2019 review: the same figure as for non-pregnant adults. If a clinician has told you to restrict sodium for a specific reason, follow that instead.

Does magnesium supplementation prevent preeclampsia? The Cochrane review of magnesium supplementation in pregnancy found there is not enough high-quality evidence to show it is beneficial. When analysis was restricted to high-quality trials, none of the primary outcomes (including preeclampsia) differed significantly between supplemented and control groups.

Is magnesium sulfate the same as a magnesium supplement? No, and conflating them is a meaningful error. Magnesium sulfate is an intravenous medication given under monitoring in hospital to treat preeclampsia and eclampsia. An oral magnesium supplement is a different substance at a different dose by a different route, and the evidence for the hospital treatment does not transfer to the supplement.

Why do fluid needs increase so much in pregnancy? Largely because blood plasma volume expands dramatically. Measured against the non-pregnant baseline, plasma volume rises about 6% by weeks 7 to 13, 29% by weeks 21 to 27, and 48% (roughly 1,150 mL) by weeks 35 to 38. That expansion is normal and necessary, and it has to be filled.

Keep Reading

• The Signs of Dehydration That Don't Involve Feeling Thirsty: the general warning-sign checklist, useful alongside the pregnancy-specific flags above.
• Your Real Daily Water Number, Calculated Step by Step: the general adult calculation, to discuss with your provider rather than apply directly.
• What to Add to Your Water: Every Category, Evidence-Graded: how supplement categories hold up generally, none of which is pregnancy-specific.

A Note on Scope

This article is for informational purposes only and is not medical advice, and pregnancy is a context where that distinction matters more than usual. Your obstetric care provider knows your blood pressure, kidney function, medication list and history; a web page does not. Follow their guidance over anything here, particularly on fluid and sodium, where advice is genuinely individual and general targets can be wrong for a specific pregnancy. Seek care promptly for: persistent vomiting where you cannot keep fluids down, sudden or severe swelling of the face or hands, severe headache, visual changes, upper abdominal pain, reduced fetal movement, or signs of dehydration such as dizziness, confusion or markedly reduced urination. Several of those can indicate preeclampsia or hyperemesis gravidarum, both of which need medical assessment rather than more water.

Sources

• Plasma volume expansion in pregnancy: a systematic review: the trimester-by-trimester plasma volume increases with confidence intervals, the 48% / 1,150 mL peak, and the rate-of-gain finding in early second trimester.
• National Academies: Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate: the 3.0 L/day total water intake for pregnancy, and the definition of total water as including food and all beverages.
• National Academies: "Sodium and Potassium Dietary Reference Intake Values Updated in New Report" (2019): the reaffirmed sodium adequate intake for ages 14–50.
• Makrides M, et al. "Magnesium supplementation in pregnancy." Cochrane Database of Systematic Reviews: the insufficient-high-quality-evidence conclusion, and the finding that no primary outcome differed when restricted to high-quality trials.

Not used: specific numeric thresholds for when vomiting in pregnancy constitutes hyperemesis gravidarum, and a specific daily cup-count recommendation attributed to ACOG. Neither could be read from its primary source, so this guide describes the warning signs qualitatively and uses the National Academies intake figure instead. Also excluded: a claim that magnesium supplementation reduces preeclampsia risk by roughly 30%, which does not reflect the Cochrane analysis cited above.