Pregnancy Weight Gain Calculator · Practical guide
How Much Weight Should I Gain by 20 Weeks?
At the halfway point, expected gain ranges from about 3 to 17 pounds depending on your pre-pregnancy BMI.
At a glance
The Answer for a Normal BMI
By week 20, a woman who started pregnancy at a normal weight should typically have gained 5 to 13 pounds: the 1–5 lb expected by the end of the first trimester, plus 0.5–1 lb for each of the eight weeks since.
Interactive tool
Pregnancy Weight Gain Calculator
Enter your own values, then compare the result with the examples below.
Weight Gain Recommendations
Week-20 Ranges for Every BMI
| Pre-pregnancy BMI | Weekly Rate After Week 12 | Expected Gain by Week 20 |
|---|---|---|
| Underweight (below 18.5) | 1.0–1.5 lb | 9–17 lb |
| Normal (18.5–24.9) | 0.5–1.0 lb | 5–13 lb |
| Overweight (25–29.9) | 0.4–0.6 lb | 4.2–9.8 lb |
| Obese (30+) | 0.3–0.5 lb | 3.4–9 lb |
The Math Behind the Ranges
The calculator builds each range from two pieces: a first-trimester base of 1–5 lb through week 12, then the weekly rate multiplied by the weeks since. At week 20 the formula is simply 1 + 8 × low rate to 5 + 8 × high rate. For a normal BMI that is 1 + 8 × 0.5 = 5 lb at the low end and 5 + 8 × 1.0 = 13 lb at the high end.
If You Are Outside the Range
Landing a pound or two outside the range at one weigh-in is common — fluids and meal timing move the scale. What matters is the trend over several weeks. If you have gained almost nothing by 20 weeks, or far more than the top of your range, mention it at your next prenatal visit rather than changing your diet on your own.
Frequently Asked Questions
Is 20 weeks really the halfway point?
Yes — a full-term pregnancy runs 40 weeks from the last menstrual period, so week 20 is the midpoint and when the anatomy scan is usually done.
How much does the baby weigh at 20 weeks?
Roughly 10 ounces — well under a pound. Most of your gain at this stage is placenta, fluid, blood volume, and maternal stores, not the baby itself.
Why have I barely gained anything by 20 weeks?
First-trimester nausea, food aversions, or a naturally slower start can all delay gain. Bring it up with your provider, who may check growth with an ultrasound.
Will I gain faster in the second half of pregnancy?
The weekly rate stays roughly the same, but the absolute numbers climb steadily — a normal-BMI mother typically goes from 5–13 lb at week 20 to 25–35 lb by delivery.