Pregnancy weight gain by week

Weekly weight gain during pregnancy doesn't follow a single rate — it changes dramatically by trimester. The first 13 weeks see almost no gain (1-4.5 lb total). After that, the IOM 2009 guidelines recommend a steady weekly target that depends on your pre-pregnancy BMI.

This guide breaks down what to expect each week from week 1 to week 40 — with weekly rates by BMI category, where the weight actually goes, and how to handle weeks where your scale doesn't match the chart.

Quick reference: weekly gain by BMI

The IOM's recommended weekly weight gain rates apply after the first trimester (from week 14 onwards). Use your pre-pregnancy BMI as the reference point:

Weeks 1–13: First trimester (almost no gain)

The first trimester is unique — total expected gain is just 1–4.5 lb (0.5–2 kg) across all 13 weeks. That's roughly 0.1–0.3 lb per week, but most women don't see linear progression. Many lose weight in weeks 6–10 due to morning sickness, food aversions, or fatigue.

What's happening week by week:

• Weeks 1–4:No measurable gain. You aren't technically pregnant for weeks 1–2 — pregnancy is counted from LMP.
• Weeks 5–8: Possible 0–1 lb gain. Morning sickness often peaks around week 8–9.
• Weeks 9–13: Gain of 1–3 lb is typical. Appetite usually returns by week 12–13 as nausea fades.

Don't panic if you're losing weight in the first trimester. As long as you're hydrated and getting some nutrition, the baby's tiny size (less than 1 oz at week 13) means your body has plenty of reserves to support development.

Weeks 14–26: Second trimester (~1 lb/week for normal BMI)

The second trimester is when steady, predictable weight gain begins. For normal-weight women, the IOM recommends approximately 0.42 kg (~1 lb) per week. Total expected second trimester gain: about 12 lb / 5.5 kg over 13 weeks.

Week-by-week milestones:

• Weeks 14–17: Appetite often surges. Gain of 3–4 lb in this 4-week stretch is normal.
• Weeks 18–22: Anatomy ultrasound around week 20. Baby is growing rapidly. Continue ~1 lb/week.
• Weeks 23–26: Belly is now visibly pregnant. Maternal blood volume peaks. Steady ~1 lb/week.

Add about 340 calories/day in the second trimester (above your pre-pregnancy needs) — equivalent to a yoghurt + a piece of fruit + a slice of toast. Not a giant meal.

Weeks 27–40: Third trimester (~1 lb/week, slowing near term)

The third trimester continues at roughly the same weekly rate as the second — about 1 lb per week for normal-weight women. However, the pattern often shifts in the final 2–4 weeks.

Week-by-week pattern:

• Weeks 27–30: Steady ~1 lb/week. Baby gaining ~0.5 lb/week. Amniotic fluid still increasing.
• Weeks 31–34:Continued ~1 lb/week. Baby's growth accelerates — fat layers building.
• Weeks 35–37: May slow to 0.5–1 lb/week as stomach gets compressed. Amniotic fluid plateaus around week 36.
• Weeks 38–40: Often very little gain or even slight loss. Body is preparing for labour. Some water retention may release.

Add about 450 calories/day in the third trimester — but listen to your body. Some women feel less hungry as space tightens near the diaphragm.

Where the weight goes (week by week breakdown)

For a normal-weight woman gaining 30 lb total, here's how the weight builds across pregnancy:

• Baby: 7–8 lb (most growth in weeks 27–40)
• Maternal fat & nutrient stores: 6–8 lb (gradual, weeks 14+)
• Increased blood volume: 3–4 lb (peaks around week 32)
• Increased fluid retention: 2–3 lb (mostly third trimester)
• Larger breasts: 1–3 lb (early second trimester onwards)
• Placenta: 1.5 lb (full size by week 36)
• Amniotic fluid: 2 lb (peaks around week 36)
• Larger uterus: 2 lb (gradual throughout)

Weeks where the scale doesn't match the chart

The IOM weekly rates are averages. Real pregnancy weight gain almost never matches a perfectly straight line. Common weekly variations that are still normal:

• Gaining 2–3 lb in one week, then 0 lb the next
• Slight weight loss during a stomach bug or stress week
• Big jumps after a salty meal (fluid retention)
• Slower gain weeks 38–40 as the body prepares for labour

What matters is the trend over 4 weeks, not any single week. If your 4-week average is within ±50% of the IOM target for your BMI, you're on track.

When to talk to your provider

Speak to your healthcare provider if you notice any of:

• Sudden weight gain >3 lb in one week (could indicate preeclampsia)
• Weight loss in the second or third trimester
• No weight gain for 3+ consecutive weeks (after week 14)
• Total gain significantly above or below the IOM range for your BMI
• Sudden swelling in face, hands, or feet alongside weight gain

Don't try to lose weight while pregnant unless specifically directed by your provider — caloric restriction can affect fetal development.

Tracking tips for accurate weekly data

• Weigh yourself the same day each week (e.g., every Monday morning)
• Use the same scale, ideally before eating or drinking
• Wear similar clothes (or none) for consistency
• Don't obsess over daily fluctuations — weekly is the right granularity
• Keep a simple log: week number, date, weight, total gain