Maternity sizes: why you keep your usual size

Maternity clothes are sized to the body you had before pregnancy. A woman who wore a medium takes a maternity medium, all the way through: the extra room for the bump is part of the garment, not a bigger size. That is what the maternity cut is for.

The exception is bras. A maternity or nursing bra has to fit the body you have now, so it goes by fresh measurements of the underbust and bust.

How a maternity size makes room

A maternity garment keeps the parts of a regular pattern that change least in pregnancy (shoulders, arms, back) and adds room where the body grows:

• Extra length and width at the front, so a hem doesn't ride up over the bump.
• A stretch panel or a fold-over waistband, worn over the bump or under it.
• Ruching, side gathers or ties that let out as the bump grows.
• Stretch jersey in tops and dresses, so one size follows the changes.

So the size label tracks your frame, and the cut tracks the pregnancy. Going up a size in maternity wear mostly buys longer sleeves and wider shoulders, because the room for the bump is already there in your usual size.

The Maternity template, next to the Dresses one

Ruban Size's Maternity template uses the same letters and US sizes as its Dresses template, with fuller numbers for the same size. Body measurements, in cm:

At every size, the maternity bust sits 4 cm above the dress bust and the hips 2 cm above. Same label, fuller body. And there is no waist row at all: the waist is the measurement that changes most, so it says little about which size to take. The template's note puts the rule in one line: "Choose your pre-pregnancy size: the fabric grows with your bump."

Try it with a bust and a hip measurement:

What to measure, and when

For clothes: start from your pre-pregnancy size. If you don't know it, or the brand's chart is new to you, measure your bust and your hips (feet together, around the fullest part: how to measure your hips) and compare them with the brand's maternity chart, not its regular one.

For bras: measure again, and measure again later. The Maternity and nursing bras template says when: "Measure from the second trimester: the band grows with your ribcage, the cup with your bust." Take the underbust snugly around the ribcage, just under the bust, and the bust around its fullest part, tape level. The band comes from the underbust, the cup from the difference between the two (how to measure your bra size has the steps).

Because band and cup can change at different rates, the sister sizes are worth knowing: one band down and one cup up, or one band up and one cup down, hold the same cup volume. A bra that fits the cup but feels tight in the band has a sister one band up.

What a maternity store should show

1. The rule, at the top of the chart. "Choose your pre-pregnancy size." It answers the first question every shopper has.
2. Body measurements without the waist, or with a waist row clearly labeled as pre-pregnancy.
3. How the bump fits. "Over-bump panel" or "under-bump waistband", and how far it stretches, if you know it.
4. Bras on their own chart, with band and cup, and the advice to measure again.
5. One chart per kind of garment. Maternity jeans, dresses and bras are cut differently, and a shopper should get the chart of the product she's looking at.

How Ruban Size handles maternity

Two templates to start from. The Maternity template has XS to XL with US and EU sizes, bust and hips, and the pre-pregnancy note. The Maternity and nursing bras template has bands 75 to 90 with cups B to E, each band merged across its cups, and the note about measuring from the second trimester. Both open as drafts with "Typical values: adjust them to your brand's own measurements before publishing." Compare them with the regular Dresses template to see where your own maternity grading should sit.

The right chart on each product. A rule like "title contains Maternity" or a maternity tag attaches the maternity chart to every maternity product, and keeps it off the rest. The product rules list the matching products before you save, and exclusions keep a maternity bra from also showing the maternity dress chart.

Find my size, from the measurements she has now. On the maternity chart, Find my size compares bust and hips and gives one size, with a line per measurement and how sure it is. On the bras chart, it finds the band and cup, gives the same size in seven systems (EU, FR, IT, UK, US, AU and JP) and the sister sizes, and can work out an EN 13402 size from underbust and bust alone.

Nothing leaves her phone. Measurements typed into Find my size are saved on the shopper's device only, never sent to Ruban Size or to the store. For a pregnant shopper entering her measurements every few weeks, that matters, and she can clear them with one tap ("Forget my measurements").

Questions

Do I buy maternity clothes in my normal size?

Yes. Maternity clothes are sized to your pre-pregnancy size: if you wore a medium, take a maternity medium. The room for the bump is built into the garment.

When should I start wearing maternity clothes?

When your regular clothes stop fitting comfortably at the waist or the bust. There is no fixed week; the bump, not the calendar, decides.

Does bra size change during pregnancy?

It can, in both the band and the cup: the band grows with the ribcage and the cup with the bust. That is why maternity bra charts ask you to measure again, from the second trimester, rather than use your usual bra size.

Should I size up in maternity clothes?

Usually not. Sizing up adds width at the shoulders and length in the sleeves, while the bump already has its room. Size up only if your bust or hips fall above the brand's maternity chart for your usual size.