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This blog is aimed, specifically, at teaching students of fashion design how to make close-fit stretch-wear patterns. While the public can certainly learn a lot from reading the blog, they may find they need the added guidance of an "in class" fashion teacher ... I'm not going to provide this level of instruction.

Everything you need to design women's swim or dancewear patterns is already here. By combining the various elements of each lesson a design student should be able to create any number of designs. I will not be adding new patterns unless it becomes necessary for one of my classes.

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The Maternity Block

A maternity swimsuit is made by making modifications to a one piece block that would have fit correctly prior to the woman becoming pregnant. This is because the majority of measurements that affect swimsuit patterns don’t significantly change. No way I hear you say? Everything gets bigger you say? Wrong. With the exception of the pelvis opening a little toward the term end, the skeleton doesn’t change one bit. The tummy gets bigger … no arguement there. Bust gets bigger … well most of the time yes but not always, and may not change significantly until the last couple of months. Besides, a woman may, for example, go from a 10B to a 10DD … but what’s to say another unpregnant woman with the same body measurements might have a 10DD bust?? What I mean, is that the bust is a seperate cup size issue and not part of the maternity considerations immediately.

But what about swollen ankles, fat thighs, sagging arms and pretty much retained fluids everywhere?? … you can tell I’m a father can’t you? Well, with the exception of the bigger tummy, bust and maybe a little extra on the bottom, the swimsuit doesn’t really care about the rest. You create the block to the prematernal body measurements … swapping the measurements for the larger bust if necessary … and then we modify it for the tummy, bottom (if needed) and make any modesty considerations like a more conservative leg line. Bust support modifications are no different than working with larger busts to begin with, except that you don’t want to be poking and forcing tender breasts with underwires (especially if you hope to get a few months wear out of one swimsuit on a rapidly changing body) … full coverage empire line shelf bras without any cup definition are the most comfortable after breasts start changing.

Take a look at the image for 0 months in the illustration below. The cross section through the torso shows a green line at the underwire line above which there is no change besides cup size (I just know people are having a hard time thinking this way but try to stick with me). The blue line represents the position of the pubis bone at the front of the pelvis below/behind which there is no change (even if the cheeks change size the line between them in cross section does not!). It is the distance between these two points that we are the most interested in as pregnancy progresses. Measuring this requires obvious discretion so, really, stop around 5cm from the pubis bone and add the 5cm to your measurement … the line is also around 5cm from the crotch line for a size 10.



Now take a look at the image for 3 months. The red line represents where the torso has changed to date (note that every woman is different … this is an average/approximate). The original body shape is still shown by the light grey line. There is also often some waist increase front on, but it is not considerable in this view. The dashed line represents where the most change from the side view is situated in the front view.

I’d like to point out now that all this is with respect to the average body ‘shape’ below size 14-16. Above these sizes, or for people carrying extra weight, you may not even notice any changes yet … no matter, the standard one piece block will be fine for a while longer. I ignore big/small, tall/short or fat/thin and simply take the measurement from green line to blue line every time my client presents for a new swimsuit, assuming she came in for a prematernal fitting to begin with … if she didn’t I need to start guessing.

Even a quick view of the last two images will show you the distance between the green and blue lines is increasing, as is the front view of the waist and the size of the bump. You might also note that the back is more arched. So how much is it exactly? Home sewers get out a tape and measure this line when you first find out you’re pregnant! Students and designers/pattern makers will need to consult the table below … which I wish to stipulate yet again is an average I’ve developed by extrapolating from a relatively small sample of women (less than 50) … it has been reasonably accurate in predicting so far so I’m happy to publish it. Indivdual women will vary from this, but if you plot their progress on the table then you’ll be able to predict the next size they’ll need … which is especially useful for multiple births!! The purpose of the table is to assist with ready to wear pattern making … if you follow it you should be about right for at least 50% of women (yes pregnancy is that unpredictable).


Size Pubis to Crotch (cm) Distance between Underbust (green) and Pubis (blue) for each Month of Pregnancy (cm)
0 1 2 3 4 5 6 7 8
8 4.8 29 29 29.1 29.5 30.5 31.9 33.4 34.8 36.3
10 5 30 30 30.1 30.5 31.5 33 34.5 36 37.5
12 5.2 31 31 31.1 31.5 32.6 34.1 35.6 37.2 38.7
14 5.4 32 32 32.1 32.5 33.6 35.2 36.8 38.4 40
16 5.6 33 33 33.2 33.6 34.6 36.3 38 39.6 41.3
Front Waist   0% 0-5% 0-15% 5-20% 15-25% 25-30% 40% 50% 60+%
Back Waist   0% 0% 0% 5% 10% 15% 20% 20% 20%


Ignoring any increase in waist for a moment, what should become immediately apparent is that nothing really happens for quite some time. A five month pregnant woman can probably fit into her normal one piece. What I find is that while they only just fit, it’s obvious they’re pregnant so they’re also starting to look for a more conservative style …. so why not make it fit better anyway? Most ready to wear swimwear designers aim at the 5-7 month range for maternity swimsuits, add in an average cup size larger for the shelf bra, and lower the leg line. The theory is that if she shows too much earlier on she’ll buy it a 4 months and that after 7.5 months she’s not really going to want to swim anyway if she’s too big to squeeze into their poorly sized 5-7 month garment. For my custom swimwear clients I make a slightly conservative swimsuit at 3-4 months if they’re showing enough, a second and more conservative swimsuit at 5 months, and a third even more conservative and supportive swimsuit at 7 months if they’re still interested in swimming (which they really should be doing for their health anyway). The last garment is usually much more about function than show stopping style!

But let’s get back to that extra around the waist? In the table above there are two rows for front and back waist. The percentages represent the increase, from starting waist size, from side seam to side seam as pregnancy progresses. You’ll notice this is a really wide and variable range but at least sizes 8-16 fit within it … generally the bigger you start the less the increase but still that has too many exceptions from which to draw a reasonable general conclusion … sorry but I can’t give you better than this as I just don’t have the data to support it. As soon as I make friends with a midwife you can bet I’m going to ask! The 5-6 month period is, however, reasonably consistent enough from which to design a ready to wear range. If you’re making a custom swimsuit, get out the tape measure.

So, armed with the above information let’s have a go at creating a 5-month maternity block from the one piece block we created earlier. I’m going to base this demonstration on the same 10B/C block and make allowance for a slight increase in bust size.

Step One

Trace out the one piece block and, if desired, lower the leg line as described in the 70′s square leg maillot. I’ve actually lowered it a bit further still just at the side seam and recurved the seam to soften it a little more. If you want to allow for a little extra in the bust, open the front block 1cm at the bust line and redraw the dart as shown. I’ve also extended the dart an extra 1cm to the side seam and recurved the seam. This will be covered in detail in the section on modifying the block for a larger bust (larger size issues – part three) … but is straight forward enough to illustrate here.

Step Two

For a little extra clarity I’ve since broken this step into two parts.

For a size 10 at 5 months we’re looking to increase the center front by 3cm at the waist line. Cut the front block at the waist and open by 3cm. Cut the back block at the waist and open up a dart half the front amount, in this case 1.5cm (this is supposed to be called a wedge rather than a dart, but the term seems to have gone missing these days).

If you look at the waist lines you’ll notice that they are wider … the front is 25% wider and the back waist is 15% wider as per the table above. Calculate in percentages so you maintain your negative ease exactly.

OK, now let’s fill in the gaps. Draw in the back wedge to the width of the new waist as shown. To maintain the length of the side side we need an wedge in the front block that’s only 1.5cm (half of the 3cm) at the side and 3cm at the center front … but if you look at the front wedge you’ll notice I don’t start to narrow the wedge until about 10cm from center front.

The result is two little darts either side of the front wedge.


Step Three

The new waist line is not perfectly center of these darts as you would expect. Instead I use the lowest edge of the wedge as the new waist line because the bulk of the ‘bump’ is actually in this position. If you take the two little darts and add them together then place them over the new waist line (still ending 10cm from center front) you should get what you see in the illustration.Recurve the center back (which is now no longer foldable) and extend the center front as shown. Make sure the side seam lengths still match, opening/closing the front side seam dart to account for any changes in length (there really shouldn’t be any unless the woman had a very square waist to begin with).

Step Four

Remove unnecessary guidelines, points and numbers. Cut out or retrace the front and back blocks. Make sure you clearly label your block with a title (including maternity term), panel name, size, date, author’s name and version number. Do not add seam allowance to the block! That’s really all there is to it. When it comes to making the pattern you could lose the dart in a princess line or simply ease the the front panel into the back over 10cm either side of the dart … its really up to you … but don’t take the dart out of the block … it needs to be there!


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