Ma'am, you are between sizes at an 18MP & 20MP. Without seeing you, I would suggest going larger, as you can always have the skirt taken in. Whereas with the smaller, there is not much room to let out, and alterations tend to be more expensive with when letting out skirts, due to removing and adjusting the length of the waistband.

Yes, Ma'am. We are happy to help.
We invite you to send your height & weight, along with your chest, waist, and hip measurements to service@marlowwhite.com. We will view your sizes and provide you with the best option based off the information provided.

Look at the inside label of your skirt or trousers, that label will have the fabric content listed.
Professional: 100% Polyester
Classic: 55%/45% Poly/Wool
Premium: 55%/45% Poly Wool Elastique