I also got the same weight gain range. But I think it is based upon your BMI rather than your exact height/weight. Also, I think the babies weight is just an average. I wanted to see, so added an extra 15 kgs to my weight and it said I would only gain 6 kg, but the babies details didn't change.
You will probably gain 6.8 kgs during your pregnancy.
If you gained the average of the range above, this is where the weight would go (totals are rounded):
You:
Uterus 0.4 kgs
Breasts 0.1 kgs
Blood 0.5 kgs
Water 0.6 kgs
Fat 1.3 kgs
Subtotal 2.9 kgs
Your baby:
Fetus 3.4 kgs
Placenta 0.2 kgs
Amniotic Fluid 0.3 kgs
Subtotal 3.9 kgs
Total 6.8 kgs
will be interesting to see how much i end up gaining
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