How to use it
Enter the first day of your last menstrual period (LMP). Count the first day of actual bleeding, not spotting.
Enter your average cycle length in days.
Read the result: your estimated due date, how far along you are today, your trimester and the key dates ahead.
If you’re not sure how long your cycle is, enter 28, which is the standard assumption. Counting back over your last few periods and averaging them will give you a more accurate figure.
Why pregnancy is counted from your last period
It seems odd, but doctors start the clock about two weeks before you actually conceive. That’s because the date of your last period is usually known, while the date of conception rarely is. So when you’re “4 weeks pregnant”, conception happened only about two weeks ago.
A full-term pregnancy is counted as 40 weeks, or 280 days, from the first day of the last period. About 38 weeks of that has passed since conception.
The formulas
With a standard 28-day cycle (Naegele’s rule):
Due date = first day of LMP + 280 days
A quick way to do this by hand is to add one year, subtract three months and add seven days.
If your cycle isn’t 28 days:
Due date = LMP + 280 days + (your cycle length − 28)
A 32-day cycle pushes the date back four days. A 26-day cycle brings it forward two. That’s because ovulation tends to happen later in a longer cycle and earlier in a shorter one.
Worked examples
A standard cycle. The first day of your last period is 1 January, and your cycle is 28 days.
A longer cycle. Same period date, but your cycle is 32 days.
The hand shortcut can occasionally land a day or two away from an exact day count, because months have different lengths. The calculator counts the days precisely.
The three trimesters
Trimester
Weeks
What it’s like
First
1 to 13
Your baby’s main organs form. Tiredness, nausea and sore breasts are common
Second
14 to 27
Often a more comfortable stretch. Many people start to feel movements
Third
28 to 40
Rapid growth, and the body prepares for birth
Some typical milestones
Timing and routine care differ between countries and providers, so use this as a general guide only.
Around week 4: a missed period, and a pregnancy test can show positive
Weeks 8 to 14: the first ultrasound, often called the dating scan. It checks the baby’s age and heartbeat
Weeks 18 to 22: the detailed anatomy scan
Weeks 24 to 28: screening for gestational diabetes is commonly offered
Week 37: the baby is considered early term
Weeks 39 to 40: full term
How likely is it that your baby arrives on the due date?
Not very. Only around one baby in twenty is born on the exact due date. Most arrive in a window of a couple of weeks either side. The usual terms are:
Preterm: before 37 weeks
Early term: 37 to 38 weeks
Full term: 39 to 40 weeks
Late term: 41 weeks
Post-term: 42 weeks or more
Think of the due date as the middle of a range, not a deadline.
Why your due date might change
The estimate from your last period assumes a regular cycle and a reliable memory of the date. Providers often compare it with an early ultrasound, which measures the baby’s size and is considered the more accurate method for dating, especially when cycles are irregular or the date of the last period is unsure. If the two disagree by more than a certain margin, your provider may change your due date. If that happens, it doesn’t mean anything is wrong.
Mistakes worth avoiding
Assuming a 28-day cycle when yours isn’t. This can throw the date off by days or even weeks.
Using a guessed date. If you’re unsure when your last period began, say so at your first appointment so an ultrasound can help.
Counting from the wrong day. Use the first day of full bleeding, not the last.
Treating the date as exact. It’s an estimate, and healthy babies are born on either side of it.
Using the calculator in place of medical care. It can’t check on you or your baby. Book your first antenatal appointment as soon as you know you’re pregnant.
Frequently asked questions
How accurate is the due date?
It’s a good estimate, but only a small share of babies arrive on the day. An early ultrasound is generally more precise than a calculation from your last period, particularly if your cycles vary.
What if I have irregular periods?
The calculator is less reliable in this case, because it assumes ovulation happens around the middle of a regular cycle. An ultrasound scan will give a better estimate. Speak to your doctor or midwife.
What if I don’t remember my last period?
Enter your best estimate, but don’t rely on it. A dating scan will settle the question.
Can I enter my conception date instead?
Not in this calculator. It works from the first day of your last period and your average cycle length. If you know your conception or ovulation date, for example from tracking or fertility treatment, ask your doctor or midwife to work out the date for you. They can also confirm it with an early ultrasound.
What if I conceived through IVF or other fertility treatment?
Your clinic can calculate the date from the embryo transfer, which is more precise than counting from a last period. Follow their date over this one.
Is the date the same for twins?
Twin pregnancies usually end earlier than single ones, often by a few weeks. Your care team will talk you through what to expect.
When should I contact my doctor or midwife?
As soon as you know you’re pregnant, to arrange your first appointment. Later, contact them without delay if you have vaginal bleeding, severe or constant abdominal pain, a severe headache, sudden swelling, or you notice your baby moving less than usual.
This calculator provides estimates for general information only and is not medical advice. Every pregnancy is different. Please speak to a doctor or midwife for personal advice, and contact your healthcare provider or emergency services straight away if you’re worried about your health or your baby’s.