Most women gain between 1 and 5 pounds during the first trimester, with an average rate of about 0.37 pounds per week over the first 13 weeks. Some women gain nothing at all, and others actually lose a few pounds due to nausea. All of these scenarios can be perfectly normal.
What’s Typical in the First Trimester
The first trimester is the slowest period for weight gain across the entire pregnancy. Research tracking pregnant women found an average gain rate of 0.169 kg (just under 0.4 pounds) per week during those first 13 weeks, which adds up to roughly 2 to 5 pounds total. The widely used IOM/National Research Council guidelines focus their specific pound-per-week targets on the second and third trimesters, not the first, because early gain is so modest and variable.
Your body isn’t yet supporting a large fetus or a fully developed placenta. The baby at 13 weeks weighs less than an ounce. Most of what you notice on the scale early on comes from increased blood volume, fluid retention, and small changes in breast tissue, not the baby itself.
Why Some Women Lose Weight Instead
Nausea and vomiting affect up to 80% of pregnant women in the first trimester, and for many, this makes gaining weight difficult or impossible. Mild weight loss of a pound or two is common and not a concern on its own.
A more severe condition called hyperemesis gravidarum goes beyond typical morning sickness. It involves persistent vomiting, dehydration, and weight loss of 5% or more of your pre-pregnancy body weight (about 7 or 8 pounds for a 150-pound woman). This condition often requires medical treatment, including IV fluids. If you’re losing weight rapidly and can’t keep fluids down, that’s worth a call to your provider rather than waiting for your next scheduled visit.
Total Pregnancy Gain by BMI Category
While first-trimester gain is loosely expected to fall in the 1 to 5 pound range regardless of your starting weight, the total recommended gain over all 40 weeks depends on your pre-pregnancy BMI. The guidelines, developed by the Institute of Medicine and endorsed by ACOG, break down like this:
- Underweight (BMI under 18.5): 28 to 40 pounds total
- Normal weight (BMI 18.5 to 24.9): 25 to 35 pounds total
- Overweight (BMI 25 to 29.9): 15 to 25 pounds total
- Obese (BMI 30 or higher): 11 to 20 pounds total
The heavier lifting happens in the second and third trimesters, when normal-weight women are expected to gain about 1 pound per week. Overweight women typically aim for about 0.6 pounds per week, and women with obesity around 0.5 pounds per week during those later months.
You Don’t Need Extra Calories Yet
One reason first-trimester gain is so minimal: your body doesn’t actually need additional calories during the first 13 weeks. The CDC specifically notes that no extra caloric intake is recommended in the first trimester. The common idea that you’re “eating for two” from day one isn’t supported by the science. Extra calories become relevant in the second trimester (typically about 340 extra per day) and third trimester (about 450 extra per day), but early pregnancy runs fine on your usual intake.
This doesn’t mean you should restrict food if you’re hungry. Many women find their appetite changes dramatically in the first trimester, swinging between nausea and intense cravings. Eating what you can tolerate, when you can tolerate it, matters more than hitting any particular calorie target.
Why Gaining Too Much Early On Matters
Gaining significantly more than expected in the first trimester isn’t just about the total number on the scale. Research published in the journal Obesity found that for normal-weight women, every standard deviation increase in first-trimester weight gain above their predicted gain was linked to a 23% higher risk of gestational diabetes. A similar pattern held for women with Class I and Class II obesity. Interestingly, second-trimester weight gain didn’t carry the same association, suggesting there’s something specific about the pace of early gain that affects how your body handles blood sugar later in pregnancy.
This doesn’t mean gaining 6 pounds instead of 4 in the first trimester guarantees problems. It means that a pattern of rapid, above-expected gain in early pregnancy is one risk factor worth paying attention to, especially if you already have other risk factors for gestational diabetes like family history or a higher starting BMI.
Where the Weight Actually Goes
By the end of a full pregnancy, the products of conception (the baby, placenta, and amniotic fluid) account for only about 35% of total weight gain. The rest distributes across your body in ways that support the pregnancy: increased blood volume (your body produces roughly 50% more blood), fluid in your tissues, larger breast tissue, an expanded uterus, and stored fat that provides energy reserves for labor and breastfeeding.
In the first trimester specifically, the gains are almost entirely on the maternal side. Your blood volume starts rising, your uterus begins growing, and breast tissue changes early. The fetus, placenta, and amniotic fluid are still tiny and contribute almost nothing to the number on your scale.
Weight Gain With Twins
If you’re carrying twins, the expectations shift upward across the entire pregnancy. The IOM recommends 37 to 54 pounds total for normal-weight women with twins, 31 to 50 pounds for overweight women, and 25 to 42 pounds for women with obesity. The weekly rate targets during the second and third trimesters are also higher: normal-weight women with twins are expected to gain 1 to 1.4 pounds per week, compared to about 1 pound per week with a single baby.
First-trimester gain with twins may run slightly higher than with a singleton, but the major differences show up later. If you’ve just learned you’re carrying multiples and are worried about early gain, the same general principle applies: a few pounds in the first trimester is typical, and the more significant gains come after week 13.