As a general rule, mummy's nipples may become more erect/swollen after pumping, hence we would highly recommend mummies to provide an allowance of at least 4mm in the selection of flanges. This is to prevent the nipple from rubbing against the nipple tunnel post-pump, etc.
If you have a nipple measurement of 18mm, the 22mm flange would be suitable. (18mm + 4mm = 22mm)
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Feel free to ping us via the live chat or email to firstname.lastname@example.org if you require any assistance in nipple measurements. We will be glad to assist you.