Can you keep the same implant size for multiple replacements?

The size of a woman’s breasts can be affected by hereditary factors, extreme weight loss, the aging process, and pregnancy.  There’s no sure way to prevent aging-related changes in your breasts.   In general after a period of time, which is variable, larger breast implants can be placed due to tissue stretch.  Two years should allow you to go bigger by at least 100 cc, maybe larger. The most important clinical factor in determining breast implant size is the base width or diameter of the patient’s native breast. If the patient would like more volume to obtain a larger cup size, the implant profile can be shifted from a moderate+ profile implant to HP implant. The average implant size placed in the US is 350cc which fall without your range of 300-375cc. 
None of the modalities to approximate final results from breast implants are 100% accurate; they merely facilitate the patient and the board certified PS concurring on the size and type of breast implant which will come the closest to reaching the patient's goal. My patients prefer to try on sizers under garments they typically wear to determine if they fit as expected and help allay some of their trepidation associated with this decision. Have another conversation with your PS and ask to try on a profile bra with 300-375 cc sizers under a tailored blouse, tight sweater, swimsuit, workout clothes, tank tops, and other garments you typically wear so that you have the best idea of whether the garments fit as desired with the implant selected.  I would recommend that you go with the size that boosts your self-esteem, and fits your anatomy.

Thank you for the good question. You are wise in considering the longer-term concerns of undergoing breast augmentation surgery. As you can imagine, every patient is different. Some patients will experience significant breast size change over the course of years; others will not. Factors such as genetics, weight gain/loss, pregnancy... vary form one patient to another. For example, some common changes seen with the breasts  after pregnancy include " involutional  hypoplasia" (breasts  become smaller)  and/or breast ptosis (" drooping"). The changes the breasts experience may be related to genetic factors, skin elasticity changes, hormonal effects, and/or weight gain/loss around the time of pregnancy. My best advice: careful selection of plastic surgeon and careful communication of goals.   I would not suggest that you "start small";  instead, aim for an outcome that you will be pleased with (understanding that long-term predictions in terms of breast size cannot be reliably made). Best wishes for an outcome that you will be very pleased with long-term.

Choose the size you want and stay with it. However, if you choose too heavy, you may need to go down in size over time. I have seen women with 40 year old saline implants that looked great at 40 years. It is possible, you may not need many replacements. If your implants are doing well at ten years and you are happy with the size, they do not need to be replaced. Only replace when needed.