Buy your pre-pregnancy size. Maternity clothes are designed to include extra room, so if your pre-pregnancy size was 12, you should aim for a 12 in maternity clothing. Sizing up is a common pitfall, giving you clothes that fit badly everywhere except the bump.
The quick answer: pick your usual size
Maternity sizing works the same way as ordinary sizing. A maternity 12 is designed with the same shoulder, bust and hip as non-maternity garments - the only difference being the bump. At Isabella Oliver we design with a typical single baby pregnancy in mind. Ordering your maternity clothes in the size you wore before you were pregnant usually covers you.
It feels counter-intuitive, which is why so many people get it wrong. You are getting bigger, so surely you buy bigger. But the room you need is in one specific place only.
Why sizing up doesn't work
Going up a size adds width more or less everywhere — shoulders, bust, thigh and hip all gain room. What it does not add is length through the front of the garment, or a rise that sits above the bump rather than under it.
So, a size-up trouser gapes at the back of the waist, sits wrong at the crotch and still digs in at the front. A size-up top hangs off the shoulders and still rides up. You end up with clothes that are too big in the wrong places and not enough in the one that matters.
Where maternity clothes add room
- Extra length at the front. The front hem is cut longer than the back so it still covers as the bump pushes it up.
- A higher or lower rise. Over-bump panels rise above the bump and hold it; under-bump waistbands sit deliberately low on the hip.
- Directional stretch. Panels stretch across and down where the growth happens, and hold their shape everywhere else.
- Side ruching and adjustable tabs. These let one garment fit across a fifteen-week span rather than a fortnight.
The three measurements that matter
If you want to check rather than assume, measure your bust, your under bust and your hips - and measure them as they were pre-pregnancy, or as close to it as you can. Once you have your measurements, our size guide gives you an idea of your size.
The one thing not to measure is your bump. The garment is not sized for it and its size will change over the course of your pregnancy. Maternity sizing is anchored to the body you had before, precisely so that it does not need re-measuring every few weeks.
When to size up, and when not to
There are three situations where going up is right; see our guide below:
If you are shorter or taller than average
Maternity blocks are drafted on a mid-height average. Height changes where the front hem and the rise land on you, which matters more in maternity than in ordinary clothing because both are doing specific work.
If you are on the petite side, over-bump styles can sit higher on the bump than intended and tops can read longer than they look on the model. On the other hand, if you are average height or taller: check front length before anything else. In both cases your size is still your size - it is the length you are checking.
The low-risk way to check
If you are genuinely unsure, or you want to test how a shape works on you before committing, renting one piece first or buying pre-loved costs less. It is also the fastest way to learn how Isabella Oliver actually fits you.
Maternity Clothing Sizing: Frequently asked questions
The takeaway
Order the size you wore before you were pregnant, check the length rather than the number if you are notably shorter or taller than average, and only size up for twins or if you sit between sizes. If you are still working out when you need any of this, we have written on when to start wearing maternity clothes and on building a maternity workwear capsule.