{"success":true,"version":"v1","request":{"tool":"get_calculator_schema","calculator_id":"pregnancy-weight-gain"},"result":{"entity_type":"calculator","id":"pregnancy-weight-gain","calculator_id":"pregnancy-weight-gain","canonical_url":"https://tttkmbb.com/health/pregnancy-weight-gain","name":"Pregnancy Weight Gain Calculator","title":"Pregnancy Weight Gain Calculator – IOM 2009 Recommended Gain by Pre-Pregnancy BMI","category":"health","category_name":"Health","tool_name":"calculate_pregnancy_weight_gain","featured_mcp_tool":false,"description":"Gives the recommended total and weekly pregnancy weight gain for a pre-pregnancy BMI according to the Institute of Medicine (IOM, 2009) guidelines for singleton and twin pregnancies, and compares the actual gain so far with the expected range for the current week.","use_when":"You need the IOM/CDC recommended weight-gain range for a pregnant woman's pre-pregnancy BMI, the expected gain by a given week, or a check of actual gain against it.","do_not_use_when":"The pregnancy is triplets or higher order, the woman is an adolescent or shorter than 157 cm (the guidelines apply with caution), or you need the due date (use pregnancy-due-date); the result is informational only, not medical advice.","inputs":[{"name":"pre_pregnancy_weight_kg","label":"Pre-pregnancy weight","type":"number","unit":"kg","required":true,"min":30,"max":300,"description":"Weight before pregnancy in kilograms.","example":60},{"name":"height_cm","label":"Height","type":"number","unit":"cm","required":true,"min":120,"max":220,"description":"Height in centimetres.","example":165},{"name":"twins","label":"Twin pregnancy","type":"boolean","required":false,"default":false,"description":"true for twins (IOM provisional twin ranges; no guideline exists for underweight women).","example":false},{"name":"current_week","label":"Current gestational week","type":"number","unit":"weeks","required":false,"min":1,"max":42,"description":"Optional week of pregnancy (from the last menstrual period) to get the expected gain so far.","example":30},{"name":"current_weight_kg","label":"Current weight","type":"number","unit":"kg","required":false,"min":30,"max":300,"description":"Optional current weight, to compute the actual gain and compare it with the expected range.","example":68}],"outputs":[{"name":"bmi","label":"Pre-pregnancy BMI","type":"number","unit":"kg/m²","decimals":1,"description":"pre_pregnancy_weight_kg / (height_cm / 100)²."},{"name":"bmi_category","label":"BMI category","type":"string","decimals":4,"description":"IOM/WHO category: underweight < 18.5, normal 18.5–24.9, overweight 25–29.9, obese ≥ 30."},{"name":"recommended_total_gain_low_kg","label":"Recommended total gain (low)","type":"number","unit":"kg","decimals":1,"description":"Lower end of the IOM 2009 total-gain range."},{"name":"recommended_total_gain_high_kg","label":"Recommended total gain (high)","type":"number","unit":"kg","decimals":1,"description":"Upper end of the IOM 2009 total-gain range."},{"name":"recommended_total_gain_lb","label":"Recommended total gain (lb)","type":"string","decimals":4,"description":"The same range in pounds as published by the IOM, e.g. '25–35 lb'."},{"name":"weekly_rate_low_kg","label":"Weekly rate, 2nd/3rd trimester (low)","type":"number","unit":"kg/week","decimals":2,"description":"IOM mean rate of gain in the second and third trimesters (for twins: derived from the total range over weeks 14–40)."},{"name":"weekly_rate_high_kg","label":"Weekly rate, 2nd/3rd trimester (high)","type":"number","unit":"kg/week","decimals":2,"description":"Upper weekly rate for the second and third trimesters."},{"name":"expected_gain_so_far_low_kg","label":"Expected gain so far (low)","type":"number","unit":"kg","decimals":1,"description":"0.5 kg over the first 13 weeks (pro rata) plus weekly_rate_low_kg × (current_week − 13); only when current_week is given."},{"name":"expected_gain_so_far_high_kg","label":"Expected gain so far (high)","type":"number","unit":"kg","decimals":1,"description":"2 kg over the first 13 weeks (pro rata) plus weekly_rate_high_kg × (current_week − 13); only when current_week is given."},{"name":"actual_gain_kg","label":"Actual gain","type":"number","unit":"kg","decimals":1,"description":"current_weight_kg − pre_pregnancy_weight_kg (only when current_weight_kg is given)."},{"name":"assessment","label":"Assessment","type":"string","decimals":4,"description":"Recommended range, or whether the actual gain is below, within or above the expected range for the current week."}],"input_schema":{"type":"object","properties":{"pre_pregnancy_weight_kg":{"description":"Weight before pregnancy in kilograms. Unit: kg.","type":"number","minimum":30,"maximum":300,"examples":[60],"x-unit":"kg"},"height_cm":{"description":"Height in centimetres. Unit: cm.","type":"number","minimum":120,"maximum":220,"examples":[165],"x-unit":"cm"},"twins":{"description":"true for twins (IOM provisional twin ranges; no guideline exists for underweight women).","type":"boolean","default":false,"examples":[false]},"current_week":{"description":"Optional week of pregnancy (from the last menstrual period) to get the expected gain so far. Unit: weeks.","type":"number","minimum":1,"maximum":42,"examples":[30],"x-unit":"weeks"},"current_weight_kg":{"description":"Optional current weight, to compute the actual gain and compare it with the expected range. Unit: kg.","type":"number","minimum":30,"maximum":300,"examples":[68],"x-unit":"kg"}},"additionalProperties":false,"required":["pre_pregnancy_weight_kg","height_cm"]},"output_schema":{"type":"object","properties":{"bmi":{"description":"pre_pregnancy_weight_kg / (height_cm / 100)². Unit: kg/m².","type":"number","x-unit":"kg/m²"},"bmi_category":{"description":"IOM/WHO category: underweight < 18.5, normal 18.5–24.9, overweight 25–29.9, obese ≥ 30.","type":"string"},"recommended_total_gain_low_kg":{"description":"Lower end of the IOM 2009 total-gain range. Unit: kg.","type":"number","x-unit":"kg"},"recommended_total_gain_high_kg":{"description":"Upper end of the IOM 2009 total-gain range. Unit: kg.","type":"number","x-unit":"kg"},"recommended_total_gain_lb":{"description":"The same range in pounds as published by the IOM, e.g. '25–35 lb'.","type":"string"},"weekly_rate_low_kg":{"description":"IOM mean rate of gain in the second and third trimesters (for twins: derived from the total range over weeks 14–40). Unit: kg/week.","type":"number","x-unit":"kg/week"},"weekly_rate_high_kg":{"description":"Upper weekly rate for the second and third trimesters. Unit: kg/week.","type":"number","x-unit":"kg/week"},"expected_gain_so_far_low_kg":{"description":"0.5 kg over the first 13 weeks (pro rata) plus weekly_rate_low_kg × (current_week − 13); only when current_week is given. Unit: kg.","type":"number","x-unit":"kg"},"expected_gain_so_far_high_kg":{"description":"2 kg over the first 13 weeks (pro rata) plus weekly_rate_high_kg × (current_week − 13); only when current_week is given. Unit: kg.","type":"number","x-unit":"kg"},"actual_gain_kg":{"description":"current_weight_kg − pre_pregnancy_weight_kg (only when current_weight_kg is given). Unit: kg.","type":"number","x-unit":"kg"},"assessment":{"description":"Recommended range, or whether the actual gain is below, within or above the expected range for the current week.","type":"string"}}},"formula":"bmi = pre_pregnancy_weight_kg / (height_cm/100)²; singleton total gain (kg): BMI < 18.5 → 12.5–18, 18.5–24.9 → 11.5–16, 25–29.9 → 7–11.5, ≥ 30 → 5–9; twins: 17–25, 14–23, 11–19 for normal, overweight and obese; weekly rate weeks 14–40: 0.44–0.58, 0.35–0.50, 0.23–0.33, 0.17–0.27 kg/week; expected gain so far = first-trimester 0.5–2 kg × min(week, 13)/13 + weekly rate × max(0, week − 13)","method":"IOM (2009) ranges are for US women with singleton or twin pregnancies and assume 0.5–2 kg gained in the first trimester; twin ranges are provisional and have no published weekly rate, so the rate shown is derived from the total. Informational only, not medical advice.","sources":[{"name":"IOM (2009) Weight Gain During Pregnancy: Reexamining the Guidelines, National Academies Press","url":"https://www.ncbi.nlm.nih.gov/books/NBK32813/","type":"standard","retrieved_at":"2026-09-24"},{"name":"CDC – Weight Gain During Pregnancy","url":"https://www.cdc.gov/reproductivehealth/maternalinfanthealth/pregnancy-weight-gain.htm","type":"government","retrieved_at":"2026-09-24"}],"freshness":{"type":"static","max_age_seconds":null,"note":"Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output."},"examples":[{"name":"60 kg, 165 cm, singleton","inputs":{"pre_pregnancy_weight_kg":60,"height_cm":165},"expected":{"bmi":22,"bmi_category":"Normal weight (BMI 18.5–24.9)","recommended_total_gain_low_kg":11.5,"recommended_total_gain_high_kg":16,"recommended_total_gain_lb":"25–35 lb","weekly_rate_low_kg":0.35,"weekly_rate_high_kg":0.5},"url":"https://tttkmbb.com/api/v1/calculate/pregnancy-weight-gain?pre_pregnancy_weight_kg=60&height_cm=165"},{"name":"85 kg, 165 cm, week 30, now 92 kg","inputs":{"pre_pregnancy_weight_kg":85,"height_cm":165,"current_week":30,"current_weight_kg":92},"expected":{"bmi":31.2,"bmi_category":"Obese (BMI ≥ 30)","recommended_total_gain_low_kg":5,"recommended_total_gain_high_kg":9,"recommended_total_gain_lb":"11–20 lb","expected_gain_so_far_low_kg":3.4,"expected_gain_so_far_high_kg":6.6,"actual_gain_kg":7},"url":"https://tttkmbb.com/api/v1/calculate/pregnancy-weight-gain?pre_pregnancy_weight_kg=85&height_cm=165&current_week=30&current_weight_kg=92"}],"faq":[{"q":"Why is the range based on pre-pregnancy BMI?","a":"The IOM 2009 guidelines link the target gain to pre-pregnancy BMI because heavier women need less added weight for a healthy birth weight; the ranges balance the risks of small-for-gestational-age infants against caesarean delivery and weight retention."},{"q":"What if I do not know my first-trimester gain?","a":"The IOM assumes 0.5–2 kg in the first 13 weeks; this calculator spreads that pro rata over weeks 1–13 and applies the weekly rate from week 14 onwards."},{"q":"Does the calculator cover twins for underweight women?","a":"No. The IOM found insufficient data for a twin guideline below BMI 18.5 and gives none, so only the BMI and category are returned in that case."}],"tags":["pregnancy weight gain","iom guidelines","how much weight to gain pregnant","pregnancy bmi","twin pregnancy weight gain"],"related":[{"calculator_id":"bmi","reason":"BMI classification for the pre-pregnancy weight."},{"calculator_id":"pregnancy-due-date","reason":"Estimate the due date and current gestational week."}],"links":{"html":"https://tttkmbb.com/health/pregnancy-weight-gain","markdown":"https://tttkmbb.com/health/pregnancy-weight-gain.md","json":"https://tttkmbb.com/health/pregnancy-weight-gain.json","api":"https://tttkmbb.com/api/v1/calculate/pregnancy-weight-gain","schema":"https://tttkmbb.com/api/v1/calculators/pregnancy-weight-gain","openapi":"https://tttkmbb.com/openapi.json","mcp":"https://tttkmbb.com/mcp"},"version":"v1","updated_at":"2026-09-24"},"timestamp":"2026-09-24T03:51:49Z"}