Seaside Swim Bottoms Hack to Period Panties

Seaside Swim Bottoms Hack to Period Panties

Posted by Angelyn Bennett on

Hi there!  Mona here to show you how to make period panties (menstrual underwear) from the Seaside Swim Bottoms pattern. The method I explain here works for low to medium absorbency panties. It is ideal for panties that you want to wear during athletic activity, as a back-up for a menstrual cup, on low flow days, or for minor stress incontinency. 

The same steps can be used to make normal underwear with a classic enclosed gusset. For period underwear with higher absorbency I would suggest a different construction, I will say a few words about that at the end.


You can use a wide variety of lightweight knit fabrics for the main body of your panties. If your main fabric is rather thin and soft, use a more stable fabric with good recovery for the waistband, especially if you are using the crossover V piece. We want these to sit snug and safe. In mine I used a soft and drapey bamboo rayon spandex for the main pieces and a mid-weight athletic knit for the waistband.

For the gusset of your period panties you will need three types of fabric: a wicking top layer, an absorbent core layer, a leak-proof bottom layer. For each of these three layers there are a variety of options depending on individual needs and preferences. I cannot list all possible options here. You will likely have to experiment a bit to find your perfect combination.


1) A wicking top layer. This is the fabric that sits closest to your skin. It is supposed to soak up moisture and lead it to the absorbent core. I like using athletic mesh for this purpose. You can also use any fabric that you would use for the gusset liner in normal panties, for example cotton spandex, rayon/bamboo spandex, thin athletic knits.

From the Greenstyle shop, the Mist Nylon Spandex or the Soft Mesh would be good options.


2) An absorbent core layer. This is the fabric that is sandwiched between the other two layers, so not visible in the finished panties. The fabrics that can be used here are the same that are used in cloth diapers. There is a wide range from hi-tech to very natural. You can also stack several layers of the same or different fabrics. Unlike in cloth pads it is not recommended to use too thick fabrics or too many layers in period panties as the gusset would become too bulky. I use hemp fleece which has good absorbency while being thin and drapey.

 3) A leak-proof bottom layer. The most common fabric for this purpose is PUL (polyurethane laminate). A thin film of polyurethane is laminated to a woven or knit fabric, either cotton or a synthetic fibre. These fabrics are used for water-repellent outdoor garments, shower curtains, tents, diaper covers, mattress protectors, etc. For period panties I use slightly stretchy PUL-backed cotton knit.

 4) Fold-over elastic (FOE). This is needed to finish the leg edges. There are various other options commonly used in underwear (picot elastic, lace trim, bands) but for our purpose fold-over elastic works best, as it neatly encases the multi-layer gusset without adding more bulk.

Pattern preparation

It is recommended to make at least one version of the Seaside as written before attempting the period panties, to get any potential fit issues out of the way. The existing garment will also help you  prepare the pattern for our purpose.

As Seaside is meant to be made of swim knit and fully lined, I would suggest to size down for underwear.

You need the continuous front/back main piece (H1) and the waistband of your choice.

The main piece includes seam allowance along the leg edge because they are supposed to be hemmed with swim elastic. The legs of our panties are going to be finished with FOE which encases the fabric edge and does not require seam allowance. Therefore we need to cut off 3/8'' along the leg edge.


Mark the desired position of your gusset in the centre of the main piece. This is where it helps to have a finished Seaside already, as you can check where you would like the gusset to sit. You can also use a menstrual pad or liner for comparison.

Cut the pattern piece apart to make a separate front, back and gusset piece. Add seam allowances at the cut edges! I always tell myself I can add seam allowance on the fly while cutting the fabric, but five minutes later my brain has already deleted that. I've also tried writing "add seam allowance here" on the pieces with neon highlighers and I've still managed to ignore it. So I would strongly recommend to glue an actual seam allowance to the pieces!!

Create a second gusset piece: Copy the existing gusset piece. Cut off twice the width of the seam allowance at the top and bottom edge (so if you added 3/8'' seam allowance, cut off 3/4''), and cut off the width of your folded(!) FOE at the curved side edge. (So if your FOE is 3/4'' wide unfolded, that means 3/8'' folded, so that would be the width of the edge you cut off.)

In case you are confused: Yes, we created a first gusset piece where we added seam allowance, and then we created a second gusset piece where we removed it again and then some. Read on, you will soon see why :)

Cutting the fabrics



Cut the newly created front and back pieces from your main fabric.

  • Cut the first gusset piece (the bigger one) from the wicking top layer and the leak-proof bottom layer.
  • Cut the second gusset piece (the smaller one) from your absorbent core layer.
  • Not pictured here: Cut the waistband from your main fabric or waistband fabric.

 Sewing steps

The most important trick for making leak-proof period panties is that no stitches are going through all three gusset layers. There will be stitches connecting the top and absorbent layer, there will be stitches connecting the gusset to the main fabric, and there will be stitches for the leg finishing. But in none of these places will there be any thread going through the absorbent layer to the outside - which could draw moisture to the outside and cause leaks.

Some period panties tutorials make a big deal about not using pins on the PUL, but there will be lots of stitches going through our PUL anyway, this is not a problem. So use pins to your heart's content!

 1) "Quilt" the absorbent layer to the top layer.


If your absorbent layer is a fabric that sheds a lot, you can serge around the edges first. In my experience this is not strictly necessary as the absorbent layer will be encased. I have never seen any shedding from finished panties.

Place the absorbent layer centred on the wrong side of the top layer, pin or spray baste in place, stitch around the edges and in a crisscross pattern across the centre. It doesn't have to be pretty - of course you can make it pretty. We mainly want to make sure that the absorbent layer can't shift around and bunch up in there. Depending on your sewing machine it may work better with the absorbent layer on top or the other way round. The absorbent layer is non-stretchy while the top layer is stretchy, so this can be a bit tricky, take your time!

 As you can see mine looks a bit wonky - it doesn't matter if it is not entirely symmetrical, you can still trim the edges of the absorbent layer a bit afterwards. But make sure that the two fabrics are lying reasonably flat. If there are big puckers or folds, discard it and start over. Two new gusset pieces are quick to cut, and you don't want to walk around with a weird bunchy gusset later ;)

These layers will be treated as one from here on, I am calling this the inner gusset. The other piece that we cut out of the leak-proof layer is the outer gusset.

2) Assembling the gusset

There are four pieces that have to be joined in order to create the main body of the panties: the front and back are being connected with the inner and outer gusset, with the seams encased between the gusset layers so that there is a smooth finish on the inside and outside.

Left: This is what the outside of the panties will look like: main fabric right side up, and outer gusset right side up. (The polyurethan film is on the inside / wrong side of the piece. The right side is what will be the furthest away from your body.)

Right: This is what the inside of the panties will look like: main fabric wrong side up, and inner gusset right side up. (The top layer that will be next to your skin is the right side, the absorbent layer is on the wrong side.)


Cross section of the finished gusset area that we are about to create: At the gusset seams the main fabric of the front and back will be sandwiched between the inner and outer gusset, with the seam allowances enclosed. The stitches of the gusset seams go through the top and bottom gusset layers but not through the absorbent layers. The absorbent layers is quilted to the top layer.

To sew the back gusset seam, stack the pieces as follows: Outer gusset right side up. Back piece right side down. Inner gusset right side down. The absorbent layer is visible on the very top. Make sure that you are using the broader ends of the gusset pieces.

The curves will not all go in the same direction, this is correct!

Mark the centre of the edge on all three pieces, and pin or clip them carefully easing the curves into each other.


Left: Stacking the three layers with centres marked.

Centre: Use as many clips or pins as you need to hold the curved edges neatly together.

Right: What it looks like from the other side, PUL-coated side of the outer gusset visible.

Stitch with a stretch stitch or serge. I do this with a lightning stitch on the regular sewing machine as I want to keep the gusset seams as flat as possible. Depending on your sewing machine this may work better with the PUL on top or the other way round. Stick to your intended seam allowance! The seam should be equally distanced from the fabric edge and from the absorbent layer.

The front gusset seam is sewn in the same way. In order to sandwich the front piece between the two gusset layers, we need to roll the back piece out of the way, towards the back gusset seam. Keep the pieces stacked the same way as before: Outer gusset right side up. Front piece right side down. Inner gusset right side down. The absorbent layer is visible on the very top. Again the curves will not all go in the same direction, ease them in carefully.


Alternatively you can join the front and back to the outer gusset firsts (right sides together), then lay the outer gusset right side up, roll the front and back fabric up in the middle, place the inner gusset on top, right side down, and sew the two gusset seams again on the existing stitch lines.

 Turn everything right side out, it should look like this: outside with main fabric right side and outer gusset, inside with main fabric wrong side and inner gusset. The absorbent layer is sandwiched in the middle.

3) Assembling the panties

 Follow the original Seaside instructions for side seams and waist band.


4) Leg finishing

 If you have never worked with fold-over elastic before, it is well worth practicing a bit on fabric scraps.

Baste the gusset edges with a long straight stitch or long narrow zigzag within the seam allowance. Trim any dog ears, thread tails or uneven bits of fabric at the gusset seams.

Cut the fold-over elastic according to the elastic cut chart in the pattern. Our leg edges are a tad bit longer because we cut off the seam allowance, but FOE is typically softer and stretchier than swim elastic, so we can use the same measurements. You can also measure the leg edge and cut the FOE at 95% of that.

Overlap the ends of the FOE and secure with a wide zigzag stitch.

Please note: The close-up pics in the following section are from a different project where the colours and lighting conditions make the stitches easier to see.


Pin the FOE to the leg edge, wrong side of elastic to wrong side of panties, aligning the centre fold of the elastic with the fabric edge.

Do not stretch the elastic along the gusset, stretch it mildly along the front leg edge and a bit more along the back leg edge. This is not an exact science! If you are unsure, you can also quarter your leg edge and elastic, pin accordingly, then redistribute the elastic carefully so that it is not stretched along the gusset edge.

Put the join of the elastic somewhere along the back edge, not at the side seam or along the gusset, to avoid additional bulk.

For the first round of stitching the elastic to the leg edge, make sure that the bobbin thread in your machine matches the colour of your elastic. The colour of the spool thread does not matter.

With the elastic underneath the fabric, stitch with a stretch stitch (lightning stitch or narrow zigzag), taking care to keep the fabric and elastic aligned. Ideally, keep the stitch line close to the fabric edge and to the fold line of the elastic, but do not let the fabric edge slip beyond the fold line and do not stitch on the fold line. If you find this tricky, you can try to keep the stitch line a bit furthr from the fabric edge, rather centred on the elastic half that is under your fabric. Depending on your sewing machine, you will be able to align the fabric edge and elastic edge with the edges or notches of your presser foot.

The two pics below show what the result looks like from the right side and wrong side.


Now fold the elastic around the fabric edge and pin or clip to hold in place. If it doesn't want to fold neatly, that's usually because the fabric is sticking out over the fold in some places. Trim back fabric edge carefully, taking care not to cut into the seam or into the elastic.

For the next round of stitching, make sure that both threads in your machine match the colour of your elastic. Topstitch with a wide zigzag, keeping the stitch line close to the open edge of the elastic. Ideally the "zig" of the stitch should be very close to the elastic edge, but definitely not beyond it, and the "zag" should be on the centre of the elastic and not too close to the outer edge. Again try to align the edges of the elastic with your presser foot and watch that, not the needle. It really helps to test your stitch settings on scraps, adjusting the width of your zigzag stitch to the width of your elastic.

And you are done!

Making regular underwear without absorbent function

 You do not need the additional smaller gusset paper piece. Cut two layers from the larger gusset piece: one outer from your main fabric, one inner from a fabric that is breathable and feels nice on your skin. Proceed as shown.

You can also use different edge finishings like picot elastic or bands. Those do require seam allowance along the leg edge, but for example picot elastic is typically sewn with a narrower seam allowance, so you may have to cut a narrow strip off.

Making underwear with higher absorbency



The gusset as shown cannot be extended much further to the back and front than the length of a medium size pad or liner, it would feel awkward on the leg edges and would also put the absorbent layer in places where you really do not need it.

If you want to extend the gusset further in the centre front and/or centre back, you need to split the pattern piece differently, as shown here. The dotted blue lines are where I split for the liner-shaped gusset, and the solid green lines are where you would split it instead.
You can see that the lines would start around the same point at the leg edge, but would go straight up towards the waist seam. The whole centre piece would become the 3-layer gusset, and instead of joining the front and back pieces at the front and back gusset seams, you would join a left and right front piece and a left and right back piece by the same method.


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