Generally, you should order your pre-pregnancy size (in other words, the size you wore before you were pregnant). Our sizing takes into account weight gain and a growing belly all the way to 9 months. When selecting your size, use the measurements chart below as a guide.
| Pre-Pregnancy Size | 4-6 | 8-10 | 12-14 | 16-18 | 20 | 22 |
| Mermaid Maternity® Size | Small | Medium | Large | XL | XXL | 2X |
| Current Bust Measurement* (inches) | 36-37.5 | 38-40 | 40.5-42.5 | 43-45 | 46-48 | 47-49 |
| Current Hip Measurement** (inches) | 38-39.5 | 40-42 | 42.5-44.5 | 45-47 | 48-50 | 49-51 |
For larger bust sizes: If your bra cup size is DDD or higher, you may want to go up 1 size, but we do not recommend going up 2 sizes. For example, if you normally wear a size medium in a fitted t-shirt but your bust measurement falls within the 'Large' box in our size chart, you should order a size large. However, if you wear a size medium in a fitted t-shirt but your bust measurement falls within the 'XL' box in our size chart, we recommend ordering a size large, not an XL.
If you have specific fit questions feel free to contact us. When you try on your swimwear, keep in mind that it will stretch a bit when wet.
International Customers, use the international size conversion guide below to find your correct size:
| US & CANADA | EUROPE | U.K. | AUSTRALIA | JAPAN |
| 4-6 | 34-36 | 6-8 | 8-10 | 7-9 |
| 8-10 | 38-40 | 10-12 | 12-14 | 11-13 |
| 12-14 | 42-44 | 14-16 | 16-18 | 15-17 |
| 16-18 | 46-48 | 18-20 | 20-22 | 19-21 |
| 20-22 | 50-52 | 22-24 | 24-26 | 23-25 |
| 24-26 | 54-56 | 26-28 | 28-30 | 27-29 |