Pregnancy Week Calculator
Calculate your current pregnancy week and trimester from your due date or last menstrual period. Track baby development milestones.
This tool is for informational purposes only and should not replace professional medical advice. Always consult your healthcare provider for medical decisions.
How Pregnancy Weeks Are Counted
Pregnancy is dated from the first day of your last menstrual period, not from conception. So week 1 is technically the week of your last period, before you were even pregnant, and conception itself happened around week 2 or 3. By the time you miss your period and test positive, you are already considered 4 weeks pregnant. This is gestational age, the figure your midwife and notes will always use.
Fetal age (the actual age of the embryo) is roughly 2 weeks behind gestational age. So at 12 weeks gestational age your baby is about 10 weeks old in fetal terms. Both numbers are correct, they just measure different things. NHS care, scan reports, and most parenting apps use gestational age throughout.
Baby Size at Each Stage
| Week | Approximate size | Comparable to |
|---|---|---|
| Week 8 | 16mm | Raspberry |
| Week 12 | 54mm | Lime |
| Week 16 | 120mm | Avocado |
| Week 20 | 165mm | Banana |
| Week 24 | 210mm | Corn cob |
| Week 28 | 240mm | Aubergine |
| Week 32 | 280mm | Butternut squash |
| Week 36 | 330mm | Grapefruit (head only) |
| Week 40 | 360mm | Watermelon |
What Each Trimester Brings
The first trimester (weeks 1-12) covers the most rapid development. All major organs form, the heartbeat starts around week 6, and miscarriage risk drops sharply after week 12. Most early pregnancy symptoms (nausea, exhaustion, breast tenderness) peak in this stretch and often ease into the second trimester. The second trimester (weeks 13-27) is often called the easier middle: energy returns, the bump becomes visible, and you usually feel the first movements between weeks 16 and 22.
The third trimester (weeks 28-40+) is the home stretch. Lungs continue maturing, the baby gains around 200g a week from week 28 onwards, and most position into head-down by around week 32-36. Common late symptoms include shortness of breath, swollen feet, sleep disruption, and Braxton Hicks contractions. From week 37 the pregnancy is full term, and you might go into labour any time. The [pregnancy milestone calendar](/pregnancy-milestone-calendar) maps appointments across the whole pregnancy.
Frequently Asked Questions
Why do I see week and days written like '20+3'?
That format means 20 weeks and 3 days. Pregnancy notes use it because each individual day matters, particularly around scan timings. So 20+3 is more pregnant than 20+0 but less pregnant than 21+0. The calculator shows your current week along with how many days into that week you are.
Should I trust the LMP calculation or my scan date?
If you had a dating scan before 14 weeks, trust the scan date. It is more accurate than LMP, particularly if your cycles are irregular. From the scan date onwards, your due date and weekly progress should be calculated from that, not from your LMP. The calculator allows you to input your due date directly if your dating scan has already adjusted it.
Are the baby size estimates exact?
No, they are population averages and can vary by 10-15% in either direction. A baby measuring smaller or larger than the listed size for that week is usually completely normal. Concerns about growth are based on percentile tracking across multiple scans, not on a single measurement. If your growth scans show concerns, your midwife or consultant will explain what is being monitored and why.
When does the second trimester start?
The second trimester starts at week 13 (or week 14 by some definitions) and runs through to the end of week 27. The third trimester begins at week 28. Some sources mark the boundaries slightly differently, but NHS guidance generally uses 13 and 28 as the trimester transitions.
Related Tools
Pregnancy Due Date Calculator
Calculate your estimated due date from your last period or conception date. Track trimester milestones, weeks pregnant and days remaining
Pregnancy Milestone Calendar
View your pregnancy milestones week by week. Important developments, screening dates, and what to expect each trimester.
Early Pregnancy Symptoms Checker
Check if your symptoms could indicate early pregnancy. Learn which symptoms are common early signs and when to test.