I have had this question myself.
First, to answer your question, it is not inevitable, but you must be blessed with good genes to avoid these problems after pregnancy.
Remember, the most important thing is to be healthy throughout your pregnancy and take vitamins, supplements, and good nutrition in accordance with your gynecologst and obstetrician's recommendations.
Now, having said that, consider during the last weeks of pregnancy the fetus can gain half a pound per week. Therefore, a child that is delayed or late can be considerably larger than if delivered early.This can aggravate and add to the damage of the abdominal wall stretching the skin and muscles apart.
Furthermore, if you pay closer attention to the media, it appears that many stars have their children delivered via C-section. Why? I am not sure, but to me it seems as if it is disproportionately higher than the general public. It is my feeling that this is being done to control the situation and to minimize late delivery (larger children), which may be subject to greater rates of birth trauma.
Do you remember the photos of Britney Spears C-section? Go ahead and Google it. But if you look at Britney in her current videos, you rarely see the scar and she has a phenomenal belly. I believe the early C-section is in large part responsible for this. Airbrushing doesn't hurt either and this can be deceptive. If you think about all the tattoos Angelina Jolie has in her public persona, you rarely see them in her movies. So the cinematographers do an excellent job of concealing tattoos and scars as well. Don't always believe your eyes when it comes to Adobe Photoshop.
In general, remaining fit and within the dietary guidelines of your obstetrician, avoiding excessive weight gain during pregnancy, combined with the blessing of good genes and skin elasticity, and an uncomplicated pregnancy are all factors that help to minimize the potential for stretch marks, loose muscles, and localized fat that is seen after pregnancy. However, even in the best of all circumstances, these do not guarantee a wrinkle free belly, so try not to beat yourself up over it. Que sera sera.