Precision Utility
Pregnancy Week
Calculator
Duration
40 Weeks
Trimesters
3 Stages
Find out exactly how many weeks pregnant you are right now. Enter your due date or the first day of your last menstrual period and the pregnancy week calculator instantly shows your current week and day, which trimester you are in, your baby's size this week and how many days remain until delivery.
Your Dates
Current Pregnancy Week
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Trimester
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Days Remaining
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Due Date
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Conception (Est.)
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This Week's Milestone
Enter a date to begin
Your baby's size comparison will appear here
Trimester Progress
Enter a date to see your progress
Trimester Breakdown
What happens in each stage of pregnancy
1st Trimester
Weeks 1 – 12
Major organs begin forming. The heart starts beating around week 6. By week 12, your baby is about 2 inches long with all essential structures in place.
2nd Trimester
Weeks 13 – 27
Often called the most comfortable trimester. You may feel your baby move for the first time. By week 27, your baby weighs about 2 pounds and can open their eyes.
3rd Trimester
Weeks 28 – 40
Your baby gains weight rapidly and the lungs mature. By week 37, pregnancy is considered full term. Most babies are born between weeks 38 and 42.
How the pregnancy week calculator works
Choose your input mode: either the first day of your last menstrual period (LMP) or your estimated due date. If you use the LMP, the calculator counts the number of days from that date to today and divides by 7 to determine how many weeks and days pregnant you are.
If you enter your due date instead, the calculator works backwards by subtracting 280 days (40 weeks) to estimate your LMP, then calculates your current week from there. Both methods use the same standard 280-day pregnancy timeline recognised by the NHS and midwives.
The result card shows your current week and day in a clear format like "Week 24, Day 3." You also see which trimester you are in, how many days remain until your due date, the estimated conception date (approximately 14 days after LMP) and a fun baby size comparison for your current week.
The trimester progress bar gives a visual snapshot of how far along you are. The breakdown section explains what to expect in each of the three trimesters.
What you need to know about pregnancy weeks
A full-term pregnancy lasts about 40 weeks (280 days) from the first day of the last menstrual period. This is the standard used by the NHS and midwives, even though conception typically occurs around week 2.
Gestational age vs. foetal age. Gestational age counts from the LMP and is the standard measure. Foetal age counts from conception and is roughly two weeks less than gestational age. When someone says "you are 20 weeks pregnant," they mean 20 weeks gestational age.
Trimesters divide the 40 weeks into three stages. The first trimester (weeks 1–12) involves rapid early development. The second trimester (weeks 13–27) is when many parents first feel movement. The third trimester (weeks 28–40) is the final stretch as the baby gains weight and the lungs mature.
Only about 4% of babies arrive on their exact due date. Most are born within two weeks before or after. A pregnancy is considered full term from week 37, and post-term after week 42. Your midwife will discuss options such as induction as your due date approaches.
Your NHS dating scan between weeks 8 and 14 uses ultrasound measurements to confirm or adjust your due date. This is particularly helpful if your menstrual cycle is irregular or you are unsure of your LMP date.
Frequently asked questions
How do I calculate how many weeks pregnant I am?
Count the number of days from the first day of your last menstrual period (LMP) to today, then divide by 7. For example, if your LMP was 168 days ago, you are 24 weeks pregnant. This pregnancy week calculator does this automatically when you enter your LMP or due date.
Can I use my due date instead of my LMP?
Yes. If you know your due date but not your exact LMP, the calculator works backwards by subtracting 280 days from your due date to estimate the LMP, then calculates your current week from there.
What trimester am I in right now?
The first trimester covers weeks 1 through 12, the second trimester runs from week 13 to week 27, and the third trimester spans week 28 until birth around week 40. Enter your dates above to find out exactly where you are.
How big is my baby this week?
Baby size varies each week. At week 8, your baby is about the size of a raspberry. By week 20, they are the size of a banana, and by week 36, about the size of a papaya. The calculator shows a size comparison for your current week.
How accurate is the pregnancy week calculation?
The calculation is based on a standard 280-day (40-week) pregnancy from LMP. It is the same method used by the NHS and midwives. A dating scan at 8–14 weeks can provide a more precise estimate if your cycle length is irregular or uncertain.
What is the difference between gestational age and foetal age?
Gestational age counts from your last menstrual period, which is about two weeks before conception actually occurs. Foetal age counts from the estimated date of conception. The NHS uses gestational age, which is what this calculator shows.