Ideal Weight During Pregnancy, Week by Week

Pregnancy has no ideal weight. What it has is an expected gain so far, measured from the weight you were before conceiving — so the answer to “how much should I weigh at 26 weeks” is always your starting weight plus a range, never a number on its own. Enter your pre-pregnancy weight, your height and the week you are in:

What the number means in week 26

Two women can weigh exactly 160 lb at 26 weeks and be in completely different places. One started at 130 lb and has gained 30 — above the pace for a healthy starting BMI. The other started at 148 lb and has gained 12, which is on pace for hers. The scale reading is the same and carries no information by itself.

That is why prenatal care tracks a curve, not a target. The reference is your own first-visit weight, and the question at each appointment is how far the line has travelled since, not where it sits.

Expected gain by week, from a healthy starting BMI

Built from the Institute of Medicine’s Table 7-3: 0.5 to 2 kg (1 to 4 lb) across the first trimester, then 0.35 to 0.50 kg per week for a woman who started at a BMI of 18.5 to 24.9.

Week	Expected gain so far	In kilograms
13	1–4 lb	0.5–2.0 kg
20	7–12 lb	3.0–5.5 kg
24	10–17 lb	4.4–7.5 kg
28	13–21 lb	5.8–9.5 kg
32	16–25 lb	7.2–11.5 kg
36	19–30 lb	8.6–13.5 kg
40	22–34 lb	10.0–15.5 kg

The forty-week row lands just inside the published total of 25 to 35 lb for this group, which is the arithmetic checking itself.

Four starting points, four slopes

The weekly pace is set once, by the BMI you had before conceiving, and then it holds through the second and third trimesters:

Pre-pregnancy BMI	Weekly gain, weeks 13 to 40
Under 18.5	1 lb (range 1–1.3)
18.5 to 24.9	1 lb (range 0.8–1)
25 to 29.9	0.6 lb (range 0.5–0.7)
30 and over	0.5 lb (range 0.4–0.6)

A woman who began at a BMI of 31 is expected to add roughly half a pound a week — about half the pace of a woman who began at 20. Neither is dieting and neither is eating for two; the guideline simply starts them on different lines. The pregnancy BMI calculator works out which line is yours and the total range it points to.

Why no target weight is ever published

Because most of the gain is not body fat. MedlinePlus breaks a 35-pound pregnancy down as roughly 8 lb of baby, 2 to 3 lb of placenta, 2 to 3 lb of amniotic fluid, 4 lb of extra blood, 2 to 5 lb of uterus, 2 to 3 lb of breast tissue and 5 to 9 lb of fat stores. A single “should weigh” figure would have to assume every one of those, plus your starting point, and would be wrong for nearly everyone.

After delivery, the ordinary adult band comes back into use; the ideal weight calculator has it for your height. Anything unusual in the pattern — a sudden jump, or no gain across several weeks — belongs with your prenatal team, not with a chart.

This content is informational and does not replace prenatal care.