Pregnancy Weight Gain Calculator UK
Not sure how much weight to gain during pregnancy? Use our free Pregnancy Weight Gain Calculator to get a personalized guide based on your BMI and week of pregnancy.
Standard Weight Gain Guidelines
| Pre-Pregnancy BMI | Category | Recommended Gain |
|---|---|---|
| < 18.5 | Underweight | 12.5 – 18 kg |
| 18.5 – 24.9 | Normal | 11.5 – 16 kg |
| 25.0 – 29.9 | Overweight | 7.0 – 11.5 kg |
| ≥ 30.0 | Obese | 5.0 – 9 kg |
How Much Weight Should You Gain During Pregnancy?
Pregnancy is one of the most incredible journeys a woman can go through. But it comes with a lot of questions and one of the most common ones is: "Am I gaining the right amount of weight?"
The honest answer? It depends on your body.
Your healthy weight gain range during pregnancy is based on your pre-pregnancy BMI. That means two women at the same stage of pregnancy might have completely different targets and both can be perfectly healthy.
As a general guide, most women with a normal BMI should gain around 11.5 to 16 kg over their entire pregnancy. But if you were underweight before pregnancy, you may need to gain a little more. If you were overweight, a little less.
The good news is, you don't have to guess. CalZone’s free UK pregnancy weight gain calculator works it all out for you, based on guidelines from the Institute of Medicine (IoM) and supported by NICE and RCOG recommendations.
Simply enter your height, pre-pregnancy weight, current weight and your week of pregnancy. We will do the rest.
Pregnancy Weight Gain Chart by Week (UK Guide)
Not everyone gains weight at the same pace and that's completely normal. Weight gain tends to be slow in the first trimester and then picks up steadily from around week 13 onwards.
The chart below gives you a rough idea of how much weight you might expect to gain each week, broken down by your pre-pregnancy BMI category.
| Week of Pregnancy | Underweight (BMI < 18.5) | Normal (BMI 18.5–24.9) | Overweight (BMI 25–29.9) | Obese (BMI > 30) |
|---|---|---|---|---|
| Week 12 | 1.5 – 2 kg | 1 – 1.5 kg | 0.5 – 1 kg | 0 – 0.5 kg |
| Week 20 | 4 – 5 kg | 3.5 – 4.5 kg | 2.5 – 3.5 kg | 1.5 – 2.5 kg |
| Week 28 | 7 – 8 kg | 6.5 – 7.5 kg | 5 – 6 kg | 3.5 – 4.5 kg |
| Week 36 | 11 – 12 kg | 10 – 11 kg | 8 – 9 kg | 6 – 7 kg |
| Week 40 | 12.5 – 18 kg | 11.5 – 16 kg | 7 – 11.5 kg | 5 – 9 kg |
These figures are based on IoM 2009 pregnancy weight gain guidelines, which are widely used by healthcare professionals across the UK. They give a realistic pregnancy weight graph to follow, not a strict rulebook. If your weight gain looks a bit different week to week, don't panic. Fluctuations are normal. What matters more is the overall trend across your pregnancy.
Understanding Your BMI and Pregnancy Weight Gain
Your pre-pregnancy BMI (body mass index) plays a big role in how much weight you should gain. It helps your midwife or GP set a healthy target that's right for your body, not just an average number pulled from a chart.
What Is a Healthy BMI Before Pregnancy?
BMI is calculated using your height and weight. It gives a general indication of whether your weight is in a healthy range for your height. Before pregnancy, a BMI between 18.5 and 24.9 is considered normal.Once you are pregnant, your pre-pregnancy BMI is used to estimate your recommended weight gain range across all 40 weeks.
Recommended Weight Gain by BMI Category
Here is a simple breakdown of what's generally recommended, based on IoM guidelines:
Underweight (BMI under 18.5)
If you were underweight before pregnancy, you will likely need to gain a bit more weight than average. The recommended range is around 12.5 to 18 kg for a single baby pregnancy.
Normal Weight (BMI 18.5–24.9)
For most women, the recommended total weight gain sits between 11.5 and 16 kg. Aiming for around 0.4 kg per week in the second and third trimesters is a good rule of thumb.
Overweight (BMI 25–29.9)
The guidance is to gain a little less, around 7 to 11.5 kg in total. This helps reduce the risk of complications during labour and delivery.
Obese (BMI over 30)
Generally advised to gain between 5 and 9 kg during pregnancy. It's still important to gain some weight, your baby needs it.
Twin or Multiple Pregnancy
Carrying twins? The targets are higher. The National Academy of Medicine recommends a total gain of around 16.8 to 24.5 kg for twin pregnancies and approximately 23 to 36 kg for triplets.
When Do You Start Gaining Weight in Pregnancy?
Weight Gain in the First Trimester
Most women gain very little in the first 12 weeks. We are talking somewhere between 0.5 and 2 kg in total, sometimes even less. In fact, if you are suffering from morning sickness, you might actually lose a little weight early on.
That's completely normal and nothing to worry about. Your baby is tiny at this stage. The weight gain you experience in early pregnancy is mostly down to changes in your blood volume and your body starting to build up its stores.
Weight Gain in the Second Trimester
From around week 13, most women begin gaining weight more steadily, roughly 0.4 to 0.5 kg per week if your BMI is in the normal range. Your body is expanding blood volume and laying down fat stores to prepare for breastfeeding.
Weight Gain in the Third Trimester
The third trimester brings the most noticeable weight gain. Your baby is putting on weight rapidly in these final weeks, and most women gain around 0.5 kg per week during this stage.
Third trimester weight gain can feel quite sudden, but it's your body doing exactly what it should. Your baby is building fat under their skin, your amniotic fluid levels are at their highest and your body is preparing for labour
Where Does Pregnancy Weight Gain Actually Go?
A lot of women are surprised to learn that pregnancy weight gain isn't just body fat. In fact, the majority of the weight you gain goes towards supporting your baby and your body's changing needs. Here is a realistic breakdown of where that weight typically goes.
So when you look at it this way, a significant portion of your pregnancy weight gain is made up of your baby, placenta, amniotic fluid and the essential fluids your body needs to keep everything running. The fat stores are there for a reason too, they fuel breastfeeding and recovery after birth.
Research behind these figures comes from the IoM's 2009 report and the foundational work by Hytten and Chamberlain in Clinical Physiology in Obstetrics. These are the most widely cited sources used to understand maternal weight gain components during singleton pregnancies.
What Should You Eat?
You don't need to "eat for two", that's a myth. But you do need to eat well. From around the second trimester, most women need roughly an extra 200–300 calories per day.
Folate & Folic Acid
Helps protect against neural tube defects. NHS recommends 400 mcg daily until 12 weeks.
Iron
Body needs nearly double to produce more blood. Found in red meat, lentils, and spinach.
Calcium
Supports baby's bones and teeth. Dairy, fortified milks, and broccoli are great sources.
Vitamin D
Works with calcium for strong bones. NHS recommends 10 mcg daily supplement.
Protein
Essential for growth. Eggs, chicken, fish, beans, and tofu are excellent choices.
What to Avoid
Some foods carry risks during pregnancy: raw or undercooked meat, unpasteurised dairy, raw shellfish, high-mercury fish (shark, swordfish) and alcohol.The NHS has a full list of foods to avoid during pregnancy, it is worth a read if you haven't already. Want to know exactly how many calories you need each week? Try our Pregnancy Pregnancy Calorie Calculator for a trimester-by-trimester breakdown.
Frequently Asked Questions
Consult Your Doctor
This calculator is for educational purposes only and does not replace professional medical advice. Always consult your healthcare provider or midwife for personalized health plans.