The UK's most complete GLP-1 guide
Every Question Answered. Every Side Effect Explained.
- How GLP-1 medications work
- Mounjaro vs Wegovy compared
- Injection instructions
- Side effects & solutions
- What to eat on treatment
- FAQs answered
Free tracker included with every download this week
Weight Loss in the First Trimester — What You Need to Know
Weight loss during the first trimester of pregnancy is complex and requires careful medical guidance. Most UK health professionals advise against deliberate weight loss during pregnancy, even if you were overweight before conception. However, some women do naturally lose weight in early pregnancy due to morning sickness or nausea. This guide explains the medical perspective, eligibility for prescription weight management options, and how to get personalised advice from your GP or midwife about safe weight management during pregnancy.
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Calculate Your BMI Instantly
Enter your height and weight below. Supports both imperial (stone & lbs) and metric (kg). Results include NHS category, healthy weight range, and ethnicity adjustments.
What Is BMI and Why Does It Matter?
Body Mass Index (BMI) is the most widely used screening tool for assessing whether your weight falls within a healthy range. Used by the NHS, GPs, and weight management services across the UK, BMI provides a quick snapshot of your weight relative to your height. The formula is simple: your weight in kilograms divided by your height in metres squared (kg/m²).
While BMI isn't a perfect measure — it doesn't account for muscle mass, bone density, or fat distribution — it remains the standard first step in identifying potential weight-related health risks including type 2 diabetes, cardiovascular disease, high blood pressure, and certain cancers.
The NHS also recommends adjusted thresholds for people of South Asian, Chinese, or Black African heritage — a BMI of 23 or above is classified as overweight, and 27.5 as obese. Our calculator includes this adjustment as a one-click toggle.
Prescription Weight Loss Medication — UK Options and Pregnancy
Prescription weight loss medications including Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide) are not recommended during pregnancy or if you are planning to become pregnant. All three are classified as prescription-only medicines and require specific contraindications screening before prescribing.
- Mounjaro: Contraindicated in pregnancy; women must use reliable contraception during treatment and for at least 4 weeks after stopping
- Wegovy: Not approved for use in pregnancy; switch to non-oral contraception during treatment
- Saxenda: Not recommended in pregnancy due to limited safety data in humans
If you are already taking any of these medications and discover you are pregnant, contact your GP or midwife immediately. Do not stop suddenly, but seek urgent medical guidance.
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How It Works
Three simple steps to understand your weight and take action.
Enter your height and weight in our free calculator. Get your BMI, NHS category, healthy weight range, and ethnicity-adjusted thresholds instantly.
If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
Use our progress tracker to log your weight weekly, visualise your journey with charts, and stay motivated — all stored privately in your browser.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
NHS Guidance and Getting Support for Weight Management in Pregnancy
The NHS recommends against weight loss during pregnancy, regardless of your pre-pregnancy BMI or current weight. Instead, the focus shifts to adequate nutrition for both you and your developing baby. Your midwife or GP can provide personalised advice based on your individual health and pregnancy stage.
If you are overweight or obese and planning pregnancy, discuss sustainable weight management before conception with your GP. Weight loss medications like GLP-1 injections can be stopped appropriately, and lifestyle changes can be supported. After pregnancy and once breastfeeding is complete, prescription weight management options may become available again following reassessment.
- Dual GIP/GLP-1 receptor agonist
- Up to 22.5% body weight loss (SURMOUNT-1)
- Weekly injection, dose escalation over 20+ weeks
- Available via private prescription in the UK
- Doses: 2.5mg → 5mg → 7.5mg → 10mg → 12.5mg → 15mg
- GLP-1 receptor agonist
- Up to 15–17% body weight loss (STEP trials)
- Weekly injection, 5-step dose escalation
- NHS & private prescription available
- Doses: 0.25mg → 0.5mg → 1mg → 1.7mg → 2.4mg
Mounjaro vs Wegovy: Head-to-Head
A side-by-side comparison of the two most popular GLP-1 weight loss medications available in the UK.
| Feature | Mounjaro | Wegovy |
|---|---|---|
| Drug class | Dual GIP/GLP-1 agonist | GLP-1 agonist |
| Active ingredient | Tirzepatide | Semaglutide |
| Avg. weight loss | 15–22.5% | 12–17% |
| Key trial | SURMOUNT-1 (2022) | STEP 1 (2021) |
| Max dose | 15 mg weekly | 2.4 mg weekly |
| Escalation period | 20–32 weeks | 16–20 weeks |
| Administration | Weekly injection (pen) | Weekly injection (pen) |
| NHS availability | Private prescription only | NHS & private |
| Approx. private cost | £150–£250/month | £150–£300/month |
| Common side effects | Nausea, diarrhoea, constipation | Nausea, vomiting, diarrhoea |
Data sourced from SURMOUNT-1 and STEP 1 clinical trials. Costs are approximate and vary by provider. Always consult a prescriber.
Am I Eligible for Weight Loss Medication?
In the UK, eligibility for GLP-1 weight loss medications like Mounjaro and Wegovy is based on your BMI and overall health profile.
- BMI of 30 or above (obese)
- Age 18 or over
- No contraindicated conditions
- Committed to lifestyle changes
- BMI of 27 or above
- Plus type 2 diabetes
- Or high blood pressure
- Or obstructive sleep apnoea
Use our BMI calculator to check whether your BMI falls within the eligible range.
How We Built These Tools
Every calculator and guide on myweightcheck is built with accuracy and transparency in mind. Our BMI calculator uses the standard WHO formula with NHS-aligned category thresholds. Our GLP-1 projections are modelled on published clinical trial data from SURMOUNT and STEP studies.
All data stays on your device. We don't require sign-ups, and we don't store your health information on any server. Your progress tracker saves data locally in your browser using localStorage, so only you can see it.
We're based in the UK and our tools are designed specifically for UK users — supporting stone & lbs by default, referencing NHS guidelines, using NICE-approved medication data, and linking to GPhC-registered pharmacies.
Frequently Asked Questions
Common questions about BMI, weight loss medications, and our free UK tools.
Can I take Mounjaro or Wegovy if I'm in my first trimester?
No. Mounjaro, Wegovy, and all other prescription weight loss medications are not recommended in pregnancy. They are contraindicated and require reliable contraception. If you discover you are pregnant while taking these, contact your GP immediately for guidance. Do not stop suddenly without medical advice.
Is it safe to lose weight during the first trimester?
The NHS does not recommend deliberate weight loss during any trimester of pregnancy. Even if you are overweight, the focus shifts to adequate nutrition for your baby's development. Some women naturally lose weight due to morning sickness, which is different from intentional weight loss. Discuss any concerns with your midwife or GP.
I'm planning to get pregnant — can I start a GLP-1 injection now?
Yes, if you meet eligibility criteria (BMI ≥30, or ≥27 with weight-related health conditions). However, inform your prescriber that you are planning pregnancy. Most GLP-1 medications require effective contraception during treatment and for a period after stopping. Your GP can discuss the best timing for starting, stopping, and conception planning.
What weight management options are available after pregnancy and breastfeeding?
After breastfeeding is complete, prescription weight loss medications become an option again if you remain eligible and a prescriber assesses you as suitable. You can also access NHS specialist weight management services or private weight loss medication through your GP or an online clinical assessment. Your progress tracker can help monitor any changes over time.
Why aren't weight loss medications prescribed during pregnancy?
Safety data for GLP-1 and GIP/GLP-1 medications in human pregnancy is limited. These medicines slow gastric emptying and reduce appetite—effects that may not support adequate nutrition for fetal development. The precautionary principle means they are not recommended until more safety evidence exists and pregnancy is no longer a risk.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.