Gaining weight during pregnancy is normal, but you want to be sure it's a healthy amount. Healthy weight gain during pregnancy is important because you can help prevent complications such as gestational diabetes, preeclampsia, and cesarean delivery.
Most women won’t gain much weight during the first trimester because the baby is still tiny. Women are usually advised to gain between 1 to 4 pounds. Women who suffer from morning sickness may gain less or even lose weight, while other women may gain a little more due to constant pregnancy food cravings, water weight gain, or decreased activity.
It’s generally fine to not gain weight or even to lose a little in the first trimester as long as a woman gains the recommended amount during the rest of her pregnancy.
What Is a Healthy Weight Gain in Pregnancy?
The amount of weight a woman is advised to safely gain during pregnancy depends on her pre-pregnancy Body Mass Index (BMI), which is a calculation based on height and weight that can provide a better estimate of total body fat than weight alone.
Weight gain recommendations for women pregnant with one baby, per The Centers for Disease Control (CDC), are as follows in the table below.
| Pre-pregnancy BMI | Safe weight gain |
|---|---|
| Underweight: BMI less than 18.5 | 28 to 40 pounds |
| Normal weight: BMI 18.5 to 24.9 | 25 to 35 pounds |
| Overweight: BMI 25 to 29.9 | 15 to 25 pounds |
| Obese: BMI 30 or greater | 11 to 20 pounds |
Weight gain recommendations for women pregnant with twins, per The Centers for Disease Control (CDC), are as follows in the table below.
| Pre-pregnancy BMI | Safe weight gain |
|---|---|
| Underweight: BMI less than 18.5 | 50 to 62 pounds |
| Normal weight: BMI 18.5 to 24.9 | 37 to 54 pounds |
| Overweight: BMI 25 to 29.9 | 31 to 50 pound |
| Obese: BMI 30 or greater | 25 to 42 pounds |
When it comes to weight gain in pregnancy, you don’t want to gain too much or too little. Typically about one-third of women gain the recommended amount of weight during pregnancy. About one in five do not gain enough weight, and nearly half gain too much.
Gaining the recommended amount of weight during pregnancy is important because:
- Gaining too little is associated with delivering a baby who is too small
- Babies born too small may have difficulty starting breastfeeding
- May be at increased risk for illness
- May experience developmental delays
- Gaining too much is associated with delivering a baby who is too large
What Causes Weight Gain During Pregnancy?
Weight gain can be caused by many different reasons during pregnancy, including:
- Baby: 7 or 8 pounds
- Fat stores: 6 to 8 pounds
- Increased blood volume: 3 to 4 pounds
- Increased fluid volume: 2 to 3 pounds
- Larger breasts: 1 to 3 pounds
- Larger uterus: 2 pounds
- Amniotic fluid: 2 pounds
- Placenta: 1-1/2 pounds
How to Meet Pregnancy Weight Gain Goals?
A woman should determine her weight gain goals with her doctors at the start of pregnancy, and revisit them regularly throughout the pregnancy.
- Track pregnancy weight gain regularly throughout pregnancy and note how your progress compares to the recommended ranges of healthy weight gain
- Eat a balanced diet
- Consume plenty of whole grains, vegetables, fruits, low fat dairy, and lean protein
- Limit added sugars and fats found in foods like soft drinks, desserts, fried foods, whole milk dairy products, and fatty meats
- Identify calorie needs for each trimester
- During the first trimester (first three months) women do not usually require any extra calories
- During the second trimester (second three months) women usually need about 340 additional calories per day
- During the third (last three months) trimester women usually need about 450 additional calories per day
- Aim for a minimum of 150 minutes (2-1/2 hours) of moderate intensity aerobic activity (such as brisk walking) per week
From 
Pregnancy and Parenting Resources
https://americanpregnancy.org/healthy-pregnancy/pregnancy-health-wellness/pregnancy-weight-gain-968/
https://www.whattoexpect.com/pregnancy/weight-gain/