How Maternity Sizing Actually Works — and Why the Same Size Can Fit Differently Across Brands

If you've ever bought a "medium" from one maternity brand and a "medium" from another and found they fit nothing alike, you're not imagining it. There's a real, researched explanation for why maternity sizing can be difficult to predict — and understanding it can save you the frustration of assuming the problem is your body rather than the label.

Here's a finding that might surprise you: even within a single, conventional sizing system, one woman's own measurements often don't line up neatly into one size. A 2025 study published in Scientific Reports, a peer-reviewed journal from the Nature publishing group, used 3D body scans of 677 women and classified each of them using the UK Alvanon sizing system. The result: only 9.15% of the women had their bust, waist, and hip measurements all fall into the same size category. For the other 90.84%, at least one of those three measurements fell into a different size category — and 35.45% of participants were not accommodated by the Alvanon sizing scheme at all.

That's not a maternity-specific problem — those were general body measurements, not pregnant bodies. It's a reminder that a single size category is an approximation — not a guarantee that every relevant body measurement will fall neatly within the same category. Once you add pregnancy into the mix, that approximation gets stretched even further.

A 2020 study in the International Journal of Fashion Design, Technology and Education tracked 20 pregnant women with repeated 3D body scans from week 6 to week 39 of pregnancy. On average, their measurements grew by 2.7 in. (6.9 cm) at the bust, 8.8 in. (22.4 cm) at the waist, and 3.3 in. (8.4 cm) at the hip. The important point isn't that every pregnant body will change by those exact amounts — the study involved only 20 participants — but that the measured dimensions changed by substantially different amounts from each other. That makes proportional size grading a poor description of how pregnancy-related body change actually occurs. A related 3D-scanning study likewise found that body and pattern changes during pregnancy did not increase proportionally.

This helps explain why the same maternity garment may not feel or fit identically throughout pregnancy: the body dimensions relevant to that garment may be changing by different amounts.

It's a common assumption that maternity sizing chaos exists because there's no rulebook at all. That's not quite true. ASTM International publishes D7197/D7197M-21, a body-measurement table specifically for misses maternity sizes 2 to 22, meant to help manufacturers build garments around real pregnant body measurements. A general body-sizing framework, ISO 8559, also exists, though it's not maternity-specific and, by its own wording, leaves garment measurements and design ease "normally determined by the designer and manufacturers."

That last point is the real story. Standards exist, but nothing forces any brand to adopt them, or to interpret them the same way if they do. Several maternity brands we checked tell shoppers to begin with their pre-pregnancy size. But brands can use different size charts, pattern shapes, grading, garment ease, fabrics, and construction, so the same nominal starting size doesn't guarantee the same finished fit.

To see what this looks like in actual maternity shopping, we reviewed the published sizing guidance of 31 brands represented on MAMANIDA in September 2026.

Not every brand could be compared numerically. Six had no official published size chart we could locate, several published guidance without an extractable numerical table, and GLAMIX uses product-specific charts rather than one universal chart. We limited the direct measurement comparison to nine brands with a sizing band described as "Medium" or its direct equivalent and sufficiently complete numeric measurements for comparison. For eight of those brands, the figures came directly from first-party size charts; for brands using a proprietary numbering system rather than a letter size, we used only the brand's own stated cross-reference to "Medium" or the equivalent (Isabella Oliver's own UK 12–14 mapping, Légoe Heritage's own size 2 mapping), rather than imposing an external UK/US/EU conversion the brand doesn't itself state. The ninth, AIM'N, could not have its numeric table reliably extracted from its own site, so its figures were cross-checked against two independent retailer reproductions of the AIM'N chart instead — a secondary-source verification, not first-party.

Among the eight first-party brands, the midpoint of the published bust measurement at that size ran from approximately 92 cm to 100.3 cm, and hip midpoints ran from approximately 97.8 cm to 106 cm. Adding AIM'N's secondary-sourced figures extends the hip upper end to approximately 108 cm — worth knowing, since that top endpoint rests on retailer reproductions rather than AIM'N's own published table.

The charts weren't all trying to describe pregnancy in the same way, either. HATCH's chart explicitly includes a waist measurement taken over the belly at its widest point, while Légoe Heritage and Ingrid & Isabel omit a waist measurement from their sizing guidance entirely. PinkBlush publishes bust and hip figures but tells shoppers waist "will stretch to accommodate growing belly" rather than giving it a number. Isabella Oliver sizes to pre-pregnancy body measurements rather than the pregnant body at all.

That means the difference isn't simply that one brand's Medium is "bigger" than another's. Brands can define the sizing problem differently in the first place — deciding which measurements matter, which to size to, and which to treat as fixed versus elastic.

The comparison also has a real limitation worth stating plainly: some of these nine charts are maternity-specific, while others (Beyond Yoga, AIM'N) apply a regular or activewear chart to maternity pieces. HATCH's bump-inclusive chart in particular isn't measuring the same thing as a chart built on pre-pregnancy body size. For that reason, these figures illustrate how much published sizing guidance can vary — they aren't a ranking of which brand fits better, and a brand outside this nine-brand comparison isn't assumed to be more or less consistent than these.

Individual shopper accounts illustrate what maternity-size uncertainty can look like in practice. Writing on The Bump's community forums, one shopper described being "a solid medium all my life" and finding a maternity-brand medium far too tight despite having only gained a few pounds so far. On Mumsnet, a shopper who called herself "a very standard size 10, not vanity sized" found that even a size 16 in one brand's maternity jeans wouldn't fit over her thighs. A Trustpilot review of Seraphine described buying the same size from the same brand across two different pregnancies — "smaller size is too small and the same size is too big — bizarre!"

These accounts cannot tell us how common sizing problems are or establish that a particular brand systematically runs large or small. They simply illustrate the kinds of fit mismatches individual shoppers report encountering.

Treat your pre-pregnancy size as a reasonable starting point — not a guarantee. Before you buy, check the brand's own current measurement chart rather than assuming your usual letter or number will translate directly. Use the brand's published range itself to choose a size, not the midpoints referenced above — those midpoints exist only to compare charts consistently against each other, and sitting above a midpoint doesn't mean you're outside a brand's actual size range. Pay attention to construction too: stretch panels, adjustable waistbands, wrap constructions, and rigid fixed waistbands accommodate changes in fit differently. Where a brand publishes actual garment measurements, use those alongside its body-size chart rather than relying on the letter or number on the tag alone.

Because sizing varies so much between brands, comparing actual measurements and construction details — not just size labels — is one of the most useful things you can do before buying. You can compare maternity pieces across brands on MAMANIDA by size, price, and construction, so you're judging the garment itself rather than guessing from a label alone.

MAMANIDA 31-brand sizing audit — brand size charts checked (September 2026) PinkBlush (pinkblushmaternity.com) · Isabella Oliver (isabellaoliver.com) · Ingrid & Isabel (ingridandisabel.com) · HATCH Collection (hatchcollection.com) · Kindred Bravely (kindredbravely.com) · Bumpsuit (bumpsuit.com) · Légoe Heritage (legoeheritage.com) · Beyond Yoga (beyondyoga.com) · AIM'N (figures confirmed via two independent third-party retailer chart mirrors — Dancewear Centre and Prae Wellness — after AIM'N's own numeric table could not be directly extracted). The full 31-brand review, including brands excluded from numeric comparison and why, is available on request.