Pregnancy Weight Gain by Trimester — Week-by-Week Guide

First Trimester (Weeks 0 to 13)

During the first 12 weeks of pregnancy, total weight gain is typically 0.5 to 2.0 kg (1 to 4 lb) for all pre-pregnancy BMI categories. Many individuals gain very little or even lose a small amount due to nausea, food aversions, or morning sickness. The IOM guidelines consider anywhere from 0 to 4.0 kg of gain in the first trimester to be within the normal range. The foetus is only about the size of a lime at week 12 and weighs roughly 14 grams, so maternal weight gain is mainly blood volume expansion, breast tissue development, and initial uterine growth. Weekly gain in this period averages about 0 to 0.1 kg and does not yet follow the per-week rates used in later trimesters.

Second Trimester (Weeks 13 to 27)

The second trimester is when consistent weekly weight gain begins. For a normal pre-pregnancy BMI (18.5 to 24.9), the IOM recommends gaining about 0.42 kg (0.9 lb) per week, which totals roughly 6.0 to 7.0 kg over 14 to 15 weeks. Underweight individuals gain about 0.51 kg (1.0 to 1.3 lb) per week, while overweight and obesity categories gain 0.28 kg and 0.22 kg per week respectively. The foetus grows from about 90 grams at week 13 to roughly 600 to 700 grams by week 27. This is also when the uterus rises out of the pelvis and becomes visibly noticeable, usually around weeks 16 to 20. Steady weekly gain in this window is one of the strongest predictors of healthy outcomes.

Third Trimester (Weeks 28 to 40)

In the third trimester, weight gain continues at 0.4 to 0.5 kg per week for normal BMI pregnancies. The foetus grows from about 1 kilogram at week 28 to a typical birth weight of 3.4 to 3.6 kg by week 40. Additional weight comes from placental growth, increased amniotic fluid, further blood volume expansion (which reaches roughly 40% above pre-pregnancy levels), and accumulated maternal fat stores that support lactation. Total third-trimester gain for a normal BMI pregnancy is roughly 4.0 to 5.0 kg over 12 to 13 weeks. Gain rates slow slightly in the final 1 to 2 weeks as the foetus drops into the pelvis and fluid absorption shifts.

Where Pregnancy Weight Goes

Total pregnancy weight gain is distributed across several components. The baby accounts for about 3.4 to 3.6 kg at term. The placenta weighs 0.7 to 1.0 kg. Amniotic fluid contributes about 0.8 kg. Maternal blood volume increases by 1.2 to 1.5 kg, breast tissue grows by about 0.5 to 1.0 kg, and uterine tissue grows by roughly 0.7 to 1.0 kg. Maternal fat stores add 2 to 4 kg depending on pre-pregnancy BMI, and these reserves are primarily mobilized during lactation. Together these components account for the full IOM reference range of 11.3 to 15.9 kg for normal-BMI singleton pregnancies.

Warning Signs During Any Trimester

Certain weight patterns warrant discussion with a prenatal care team. Sudden gain of more than 1 kg (2 lb) per week sustained over 2 to 3 weeks can signal fluid retention and may be associated with preeclampsia, especially if accompanied by swelling of the hands and face or elevated blood pressure. No weight gain for more than 4 consecutive weeks in the second or third trimester may indicate inadequate nutrition or persistent nausea and should be evaluated. Gain more than 36% above the IOM range by week 32 is associated with a roughly doubled risk of macrosomia (birth weight above 4 kg). Consistent tracking on the same scale, ideally once per week at the same time of day, is the most reliable way to detect these patterns early.