Why do breasts sag following pregnancy?

Once the baby is weaned and the mom stops nursing, it is normal for her breasts to shrink and appear as if they have been deflated. This may sometimes become a cause of concern for the new dad.
 
These are reasons why breasts seem to sag following weaning:

• Hormonal changes because of pregnancy bring about an increase in the glandular tissues and the amount of fat in the breasts. The breasts therefore grow in size and become engorged. The tightening of the skin as it stretches to accommodate this increase may cause the breasts to appear especially voluptuous during this phase. They continue to remain enlarged during breastfeeding.
  
  
• Once lactation stops, the tissues automatically reduce in size.
  
  
• As a result, the stretched skin now becomes loose, causing it to appear as if it has sagged.

Also read: The straight story—will my wife get her breasts back after the baby?