How much to gain by this week, based on your pre-pregnancy BMI (2009 IOM guidelines)
| Pre-pregnancy BMI | Total Gain (lb) | Total Gain (kg) | Rate, 2nd–3rd tri (lb/wk) |
|---|---|---|---|
| Underweight (< 18.5) | 28 – 40 | 12.5 – 18 | 1.0 – 1.3 |
| Normal (18.5 – 24.9) | 25 – 35 | 11.5 – 16 | 0.8 – 1.0 |
| Overweight (25.0 – 29.9) | 15 – 25 | 7 – 11.5 | 0.5 – 0.7 |
| Obese (≥ 30) | 11 – 20 | 5 – 9 | 0.4 – 0.6 |
Source: Institute of Medicine / National Academy of Medicine guidelines (2009), the standard used by ACOG and most US providers.
| Pre-pregnancy BMI | Total Gain (lb) | Total Gain (kg) |
|---|---|---|
| Underweight (< 18.5) | No guideline — insufficient data; follow your OB | |
| Normal (18.5 – 24.9) | 37 – 54 | 17 – 25 |
| Overweight (25.0 – 29.9) | 31 – 50 | 14 – 23 |
| Obese (≥ 30) | 25 – 42 | 11 – 19 |
Twins pregnancies are usually monitored with ultrasound growth checks rather than weight alone; use this table as a rough frame, not a target to chase.
| Component | Typical Weight (lb) |
|---|---|
| Baby at birth | 7.5 |
| Placenta | 1.5 |
| Amniotic fluid | 2 |
| Breast tissue | 1 – 2 |
| Blood volume increase | 3 – 4 |
| Uterus growth | 2 |
| Maternal fat and fluid stores | 6 – 12 |
Roughly 7–8 lb is the baby; the rest is the support system. This is why "eating for two" at double portions is off by a wide margin: the extra need is about 300 calories a day in the second and third trimesters.
It computes your pre-pregnancy BMI from your height and starting weight, classifies it the way the 2009 IOM guidelines do, and applies the guideline's weekly pattern: about 1 to 4 lb total in the first trimester, then a steady weekly rate for the rest of the pregnancy. At any week, the recommended range is the first-trimester gain plus the weekly rate times the weeks past 13, kept inside your BMI category's total range.
BMI = 703 × weight (lb) ÷ height (in)², or weight (kg) ÷ height (m)². Then, for a singleton pregnancy at week w: recommended gain = first-trimester gain + weekly rate × max(0, w − 13). The low end uses the low first-trimester gain (about 1.1 lb) and the low weekly rate; the high end uses the high values (about 4.4 lb and the high rate). For twins, the calculator shows the total range scaled by weeks completed against 40, since the IOM publishes twin totals rather than weekly rates.
Enter your height and your weight from before pregnancy, not today's weight; the categories are defined on the starting point. Set your current week, and optionally your current weight for the on-track check. Week 14 or 40 both work; before week 14 the recommendation is simply the 1–4 lb first-trimester range. If you're carrying twins, tick the box, and remember your OB's plan outranks any calculator.
Someone 5'4" at 130 lb before pregnancy: BMI = 703 × 130 ÷ 64² = 22.3, the normal category, so 25–35 lb total and about 0.8–1.0 lb per week after week 13. At week 28 the math is 1.1 + 0.8 × 15 = 13.1 lb on the low end and 4.4 + 1.0 × 15 = 19.4 lb on the high end: about 13 to 19 lb by that point. If they weigh 149 lb at week 28, the 19 lb gained sits just inside the range, right on pace.
It depends on your pre-pregnancy BMI: 28–40 lb if underweight, 25–35 lb at a normal BMI, 15–25 lb if overweight, and 11–20 lb with obesity, for a singleton pregnancy per the 2009 IOM guidelines. For twins the ranges are higher: 37–54 lb at a normal BMI. Your provider may adjust these for your specific situation.
After the first trimester, roughly 1 lb a week at a normal pre-pregnancy BMI, 1–1.3 lb if underweight, 0.5–0.7 lb if overweight, and 0.4–0.6 lb with obesity. The first trimester typically adds only 1–4 lb total, which is why the weekly rate applies from week 14 onward.
Because the gains on top of your starting weight carry the same pregnancy, a baby, placenta, amniotic fluid, extra blood, and breast tissue, but the recommended maternal fat and fluid stores shrink as starting BMI rises. Higher starting weight is already associated with pregnancy complications, so the guideline limits additional gain to keep risk down, not to restrict the baby's growth.
Bring it up with your provider rather than cutting calories on your own; needs differ and fluid retention, multiples, and measurement timing all matter. In the second and third trimester, the commonly used shift is roughly 300 extra calories a day above pre-pregnancy intake, and the levers are usually quality of food and activity, not restriction.
No, twins get their own, higher ranges: 37–54 lb at normal BMI, 31–50 if overweight, and 25–42 with obesity. The IOM makes no recommendation for underweight twin pregnancies because the data were insufficient. Twin pregnancies are also managed more closely overall, so follow your obstetrician's numbers first.
First-trimester loss from nausea is common and usually not harmful if it's modest and you started at a healthy weight; the total-gain ranges assume you catch up as appetite returns. Tell your provider if you can't keep fluids down, the loss is more than about 5% of your body weight, or it continues into the second trimester.