Bikini First Line - Next Gen Iron-Underwear-TomboyX
Bikini First Line - Next Gen Iron-Underwear-TomboyX
First Line Leakproof Bikini - Next Gen Iron-Underwear-TomboyX

First Line Leakproof Bikini

4.4 star rating 38 Reviews
Color Next Gen Iron

It's all in the hips

For the best fit, pick your size based on your hip measurement.
Body measurement in inches

Size Waist Hips
XS 26-27.5" 33-34"
S 28-29.5" 35-37"
M 30-31.5" 38-40"
L 32.5-34" 41-43"
XL 35-37.5" 44.5-46.5"
2X 38.5-42" 48-50"
3X 43-45" 51.5-53.5"
4X 46-48" 55-56"

For more detailed information, please see our Fit Guide page.

How to measure your hips

Stand on a level surface, feet together without shoes.
Measure around the fullest part of your hips and bottom - this is usually 7-9" below your natural waistline.
Keep the tape measure parallel to the floor, and don't pull too tight.

Size chart

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Like the arid high plateaus of the Gobi desert, First Line keeps you dry and comfortable while you're on the move. With built in leak protection for light days and incontinence you can say goodbye to pads, tampons, and menstrual cups.

  • Looks and fits just like regular TomboyX Underwear
  • Holds 8 tsps of liquid
  • Odor-resistant
  • Machine wash cold in a mesh bag, line dries overnight. Avoid machine drying.
  • 95% OEKO-TEX®-certified cotton, 5% spandex
Stock Up & Save! Buy 3+ of any undies and/or bras & get 10% off. Buy 5+ and get 15% off.

Living Wage factories

Ethically produced in women-owned factories that pay living wages.


fit-tested for every body

Obsessively fit-tested on real people, sizes XS-4X.


Love at first wear

If you don't love them, your first pair is on us.


eco-friendly process

Sustainably harvested materials and earth-conscious production.

Fabrics & Technology


Our classic soft jersey knit holds its shape after many wear and care cycles.

Shop Cotton


The luxurious companion to cotton. Supersoft, breathable and stretches to meet you.

Shop Micromodal


Quick-drying, durable and ultralight performance fabric. Anti-bacterial and anti-microbial.

Shop Traveler

First Line Leakproof

Your first line of defense — our classic cotton armed with leak protection for light period days and incontinence.

Shop First Line