How much weight should I gain during pregnancy?
It depends on your pre-pregnancy BMI. Under the Institute of Medicine (now NASEM) guidelines used here, someone starting at a healthy BMI of 18.5 to 24.9 is generally advised to gain 11.5 to 16 kg (25 to 35 lb) across a single pregnancy. Underweight is 12.5 to 18 kg, overweight 7 to 11.5 kg, and obese 5 to 9 kg. Twin pregnancies have separate, higher ranges. These are population guidelines, not personal prescriptions — your midwife or doctor sets your individual target.
How is the weight gain spread across the pregnancy?
Unevenly. The first trimester adds very little, usually only 0.5 to 2 kg (1 to 4.5 lb) in total, and some people lose weight if they have nausea. Most gain happens in the second and third trimesters at a steadier weekly rate — roughly 0.35 to 0.50 kg per week at a healthy starting BMI. That is why this calculator shows a widening band across the 40 weeks rather than a single number, and plots where you sit against it.
What if I am gaining more or less than the recommended range?
It is very common and often not a problem. A single weigh-in tells you little: fluid shifts, timing, clothing and scales all vary, and gain is naturally uneven week to week. What matters is the pattern over several weeks. If you are consistently well outside the range, or the change is sudden, mention it to your midwife or doctor — sudden gain in particular can need checking. This calculator is for tracking and context, not for diagnosis.
Where does the weight actually go?
Only a fraction is the baby. At term, a typical single pregnancy accounts for roughly 3 to 3.5 kg of baby, about 0.9 kg of placenta, 0.9 kg of amniotic fluid, 1 kg of extra uterine tissue, 0.4 kg of breast tissue, 1.8 kg of extra blood volume, and around 2.7 kg of fluid and fat stores that support breastfeeding. The fat stores are a normal, functional part of pregnancy, not excess.
Do I need to eat for two?
No — that phrase substantially overstates it. Extra energy needs are modest: roughly no additional calories in the first trimester, about 340 extra per day in the second, and about 450 in the third. That is a snack, not a second meal. Nutrient quality matters more than quantity, particularly folate, iron, iodine, calcium and protein. A midwife, doctor or dietitian can advise for your situation.
Are these guidelines the same everywhere?
No. The IOM/NASEM ranges used here are the most widely cited and are standard in the United States, but some countries use different thresholds or, like the UK, do not routinely weigh throughout pregnancy at all. BMI itself is also an imperfect measure that does not distinguish muscle from fat. Treat the range as a general reference and follow the guidance your own healthcare provider gives you.