We hear a lot about big babies these days. They're the reason we do a lot of inductions, c-sections and admissions to the NICU. Babies just seem to be getting bigger. Some of that is certainly genetics. Some of it is uncontrolled diabetes and obesity. Some of it may be evolution, and we're just getting bigger as a species.
I can't tell you how many times OB-GYNs do inductions for "big" babies only to deliver normal sized kiddos. Lots. Doctors determine their estimate of fetal size based on how big your belly measures. If it measures large for dates, they figure it's going to be a big one. Other reasons for measuring large are extra amniotic fluid or abdominal fat.
Ultrasounds are better indicators of fetal size, but they're frequently way off. According to the Manual of Obstetrics, the margin of error on determining fetal size ranges between 15 and 30%. That's quite a lot, especially when you're trying to determine how best to deliver this supposedly big baby. It's always a guessing game.
But do these bigger babies result in more difficult and painful vaginal deliveries? I don't know that big-baby-labors are any more painful than average-baby-labors. Pain is such a subjective thing that I don't think we can measure accurately if one labor is more painful than another. There are too many factors like tolerance to pain, ability to relax, previous experiences with pain and/or trauma.
Whatever measures you're taking to prepare for your natural (unmedicated) labor will be good enough—including those all-important child birthing classes. Then go into labor with an open mind and hope for the best.
Like any labor, there are no guarantees that things will go as planned. Big or small, some babies don't like labor and need to be delivered by "alternative means." Though more extremely big babies are born by cesarean than others, lots and lots of big babies come out vaginally. Sure, there are likely to be more perineal tears, bruises and swelling, but they're easy enough to repair and most women recover beautifully.
The best way to deliver a healthy weight baby is to eat well, exercise and avoid excess weight gain. Extremely big babies have a harder time transitioning to life outside the uterus. They have less stable blood sugar, more respiratory and temperature control issues. They're more likely to spend a little time in the nursery with an IV and oxygen, recovering and stabilizing during their transition. Most eventually do just fine but still, no one likes it when their baby has to go to the nursery.