Assuming she is around 3-4 months along and has not gained more than 10 pounds, than the corresponding sizing guideline applies. If she has gained more than 15 pounds, then for jeans that have a numeric size (ie. 25, 26, 27 etc) consider buying an additional size larger, but consider the Brand Factor above in addition.
If she is 5-7 months along and has gained up to say 20 pounds, then you would consider ordering her pre-pregnancy size in addition to adjusting for Brand Factor. If she has gained more than 20 pounds at this stage in the pregnancy than definitely order one size larger, in addition to adjusting for Brand Factor.
The ultimate goal is to order her a pair of maternity jeans, regardless of size, that will last her through the pregnancy. So if she normally wore a size 29, and the guidance above (for both Brand and weight gain) suggests a size 31, than that is likely the ideal size.
We are always a phone call away, so don't hesitate to call us at 1-855-972-9679 or email us at firstname.lastname@example.org so that we can provide you with a personal consultation and assist with what the most accurate size would be.
And remember, if you order the wrong size we will make it right. Please see our terms and conditions section of our site for more details, https://www.sevenwomen.ca/terms-and-conditions.
Wishing you and your growing family the best!