Breast lift should not have any effect on the pregnancy process. When it comes to lactation there is always a chance that you will not be able to nurse. Lactation is a very complex process and many factors contribute to it e.g hormonal, psychological, etc..
On the other hand, pregnancy will make your breast increase in size and stretch and there is a possibility that it could sag again. The advice i give my patients is : If you are planning to have a child sometime in the next year or so, wait until you are done with pregnancy and nursing. Over the years, I found out that most patients who come for a consultation are either very motivated for this surgery or are not planning for a child in the coming few years.
Talk to your board certified plastic surgeon, he will be able to help you with a good decision.
Hope that helps.
