This LMP calculator uses your last menstrual period date to calculate everything you need to track your pregnancy — your estimated due date, your current pregnancy week and day, your AOG (Age of Gestation), which trimester you are in, and approximately how many months pregnant you are.
Enter your LMP date below and get your complete pregnancy timeline instantly — the same information your OB-GYN uses at your first prenatal visit, calculated from one simple input.
What your results mean:
Everything in this calculator is calculated from your LMP — the first day of your last menstrual period. This is the standard clinical method used in the Philippines and worldwide for dating pregnancies, estimating due dates, and determining gestational age.
Your due date is an estimate. Most babies are born within two weeks before or after the calculated date, and only around 5% arrive exactly on their EDD. Use these results as a framework, not a fixed schedule.
What is LMP — and why does it matter so much in pregnancy?
LMP stands for Last Menstrual Period. It refers specifically to the first day of your most recent period before you became pregnant.
In pregnancy care, your LMP is the single most important date your OB-GYN will ask for. Every subsequent calculation — your due date, your pregnancy week, your AOG, your trimester, your recommended prenatal visit schedule — starts from this one date.
The reason pregnancy dating uses the LMP rather than the actual conception date is practical. Most women know the date their last period started far more reliably than they know when conception occurred. Conception typically happens around 14 days after the LMP in a standard 28-day cycle — and that 14-day offset is already built into the 40-week pregnancy calculation.
This means that when you are told you are six weeks pregnant, you may have only actually been pregnant for about four weeks — because the first two weeks of the 40-week count happened before conception took place. This often surprises women who find out they are pregnant shortly after a missed period and are told they are already four to five weeks along.
Knowing your LMP date, or being able to calculate it, gives you accurate access to your entire pregnancy timeline from day one.
LMP calculation formula — how the numbers work
The LMP calculator uses three established formulas to generate your results. Understanding how these work helps you interpret what you see and know why different methods sometimes give slightly different numbers.
Due date formula — Naegele's Rule
Your estimated due date (EDD) is calculated using Naegele's Rule:
EDD = LMP date + 280 days
280 days equals exactly 40 weeks — the standard length of a full-term pregnancy. This formula has been used in obstetric practice for over 200 years and remains the standard clinical method for due date calculation worldwide, including in the Philippines.
Example: LMP of January 1 gives an EDD of October 8. LMP of July 15 gives an EDD of April 20 of the following year.
AOG formula — Age of Gestation
Your AOG is the number of weeks and days that have passed since your LMP date:
AOG = days from LMP to today, divided by 7 (weeks) with the remainder as days
Example: 59 days from LMP = 8 weeks 3 days AOG.
AOG is the term you will see most often on your prenatal record card in the Philippines. When your OB-GYN writes "8+3" on your chart, they mean 8 weeks and 3 days from your LMP.
Pregnancy month calculation
Pregnancy months are not the same as calendar months. A pregnancy month is approximately 4.33 weeks, because 40 weeks divided by 9 months gives roughly 4.33 weeks per month.
Month = AOG in weeks divided by 4.33
This is why pregnancy is described as "nine months" but is actually 40 weeks — the months of pregnancy are slightly longer than calendar months.
How to calculate your LMP if you are unsure of the date
Not everyone remembers the exact first day of their last period — especially if they were not tracking their cycle. Here are the most reliable ways to estimate your LMP when the exact date is unknown.
Work backward from a positive pregnancy test
If you know when you got a positive pregnancy test, you can estimate your LMP. Pregnancy tests typically detect hCG around 10 to 14 days after conception, which is approximately 24 to 28 days after your LMP.
Subtract 24 to 28 days from your test date to get an approximate LMP. This will give you an estimated LMP within a few days of the actual date.
Use your menstrual tracking history
If you have been tracking your cycle — even roughly — look back at when your second-to-last period started. Add your average cycle length to that date to estimate when your last period should have begun.
For example: if your previous period started on October 1 and your cycle is usually 28 days, your LMP would be approximately October 29.
Use an early ultrasound date
If you have already had an early pregnancy ultrasound, the gestational age measured by the ultrasound can be used to back-calculate your LMP.
An early first-trimester ultrasound — ideally done before 12 weeks — measures the embryo or fetal size and gives a gestational age estimate. From this, your OB-GYN can determine an LMP date equivalent, even if you do not know your actual LMP.
An ultrasound-derived LMP equivalent is often considered more accurate than a recalled LMP date, particularly for women with irregular cycles.
Pregnancy week calculator by LMP — what each week tells you
Your pregnancy week by LMP is one of the most practically useful numbers this calculator gives you. Knowing exactly how many weeks pregnant you are helps you understand what is happening developmentally, what symptoms to expect, and when your next prenatal milestones are due.
First trimester — weeks 1 to 12
The first trimester is the period of most rapid and critical development. The major organs, central nervous system, heart, and limbs all begin forming during this period.
Weeks 1 and 2 of pregnancy technically occur before conception — they are part of the LMP-based count that begins on the first day of your last period.
Weeks 3 and 4 are when fertilisation occurs and the fertilised egg implants in the uterine wall. Most women do not yet know they are pregnant at this point.
Weeks 5 to 8 are typically when early pregnancy symptoms — nausea, breast tenderness, fatigue, frequent urination — appear most intensely.
Weeks 9 to 12 mark the transition from embryo to foetus. By week 12, all major structures are formed and the risk of miscarriage drops significantly.
Second trimester — weeks 13 to 26
The second trimester is widely considered the most comfortable period of pregnancy. Morning sickness typically eases, energy levels return, and the visible baby bump appears.
Between weeks 18 and 22, most mothers feel fetal movement for the first time — often described as fluttering or gentle bubbling. The anatomy scan ultrasound, which checks fetal development in detail, is typically scheduled between weeks 18 and 20.
Third trimester — weeks 27 to 40
The third trimester is the final growth phase. The baby gains most of its birth weight during these weeks. Physical discomfort increases as the uterus grows.
Weeks 37 to 40 are considered full term. A baby born before 37 weeks is preterm. After 42 weeks is post-term.
AOG calculator by LMP — what Philippine OB-GYNs actually use
If you have been to a prenatal appointment in the Philippines, you have seen AOG written on your maternity record card. AOG — Age of Gestation — is the standard Filipino obstetric term for gestational age, and it is expressed in weeks and days from the LMP.
When your OB-GYN asks "AOG mo?" they are asking how many weeks and days pregnant you are from your LMP. The answer to that question — expressed as weeks plus days, like "10+4" or "24+0" — is what this calculator gives you directly.
AOG is used to determine appropriate fetal size, schedule prenatal check-ups, interpret ultrasound measurements, assess fetal growth, and time interventions if needed. It is one of the most frequently referenced numbers throughout your prenatal care.
The AOG number on your prenatal record also aligns with the schedule of prenatal visits recommended by the Philippine Department of Health. First visit ideally before 12 weeks AOG, subsequent visits every four weeks until 28 weeks, every two weeks from 28 to 36 weeks, and weekly from 36 weeks to delivery.
Due date calculator based on LMP — how accurate is it?
A due date calculated from your LMP is accurate to within one to two weeks for women with regular menstrual cycles of approximately 28 days. For women with irregular cycles or cycles that differ significantly from 28 days, the LMP-based due date may be less accurate.
This is because Naegele's Rule assumes a 28-day cycle with ovulation on day 14. If your cycle is regularly longer or shorter, your actual conception date may differ from the assumed date by several days to a week or more.
For this reason, OB-GYNs in the Philippines commonly use a first-trimester ultrasound to confirm or adjust the LMP-based due date. If the ultrasound measurement of the embryo suggests a different gestational age than the LMP date implies, the ultrasound date is often preferred — particularly if the difference is more than five to seven days.
The most accurate due date comes from combining your LMP date with a first-trimester ultrasound measurement, interpreted by your OB-GYN who knows your full reproductive history.
LMP calculator for Filipino mothers — practical points
Filipino obstetric practice uses the same LMP-based dating system as international standards. A few practical points are worth noting for Filipino mothers using this calculator.
Your maternity record card — the card your OB-GYN or midwife fills out at your prenatal visits — will have fields for LMP, AOG, and EDD. The numbers from this calculator match directly with what gets written on that card, making it a useful cross-reference between appointments.
If you delivered at a government hospital or birthing center, your prenatal records may use AOG notation slightly differently from private OB-GYNs — but the underlying calculation from LMP is the same across all Philippine healthcare settings.
For women who are unsure of their LMP, the best first step is a visit to your barangay health center or OB-GYN for an early ultrasound. An early ultrasound is available under PhilHealth coverage at accredited facilities and gives you a reliable gestational age even when LMP is uncertain.
Frequently asked questions
How do I calculate my due date from my LMP?
Add 280 days to the first day of your last menstrual period. This gives you your estimated due date (EDD) based on Naegele's Rule — the standard method used by OB-GYNs worldwide.
Our LMP calculator does this automatically. Enter your LMP date and your due date, AOG, pregnancy week, trimester, and pregnancy month all appear instantly.
How do I calculate my LMP from my due date?
If you know your due date but not your LMP, subtract 280 days from your due date. This gives you the approximate LMP date.
Example: EDD of October 8 minus 280 days = LMP of approximately January 1.
What is AOG by LMP?
AOG by LMP means Age of Gestation calculated from your Last Menstrual Period. It tells you how many weeks and days pregnant you are, counted from the first day of your last period.
This is the standard gestational age measurement used in Philippine prenatal care and is what your OB-GYN writes on your maternity record card at each prenatal visit.
Why does my pregnancy week seem earlier than I expected?
Pregnancy weeks are counted from your LMP date, not from the date of conception or the date you got a positive test. Conception typically occurs around two weeks after your LMP, which means your pregnancy week count includes approximately two weeks before you were actually pregnant.
This is why many women find out they are pregnant and are told they are already four or five weeks along — because the first two weeks of the 40-week count are the two weeks before conception.
How accurate is LMP-based pregnancy dating?
LMP-based dating is accurate within about one to two weeks for women with regular 28-day cycles. For women with irregular cycles, longer or shorter cycles, or uncertain LMP dates, a first-trimester ultrasound gives a more accurate gestational age.
If your LMP date and ultrasound measurements give different results, your OB-GYN will use the ultrasound date if the difference is significant — particularly in the first trimester when ultrasound measurements are most accurate for dating.
What is the difference between EDD and AOG?
EDD stands for Estimated Delivery Date — your expected due date. AOG stands for Age of Gestation — how many weeks and days pregnant you currently are.
EDD is a future date. AOG is your current status. Both are calculated from the same LMP date. Your EDD stays the same throughout your pregnancy, while your AOG increases by one day every day.
Related calculators
Your LMP is the foundation of your entire pregnancy timeline. These related tools help you use it:
Pregnancy Calculator — Get your full pregnancy timeline including due date and trimester breakdown from your LMP.
Ovulation Calculator — Understand your fertile window and ovulation timing based on your cycle.
Period Calculator — Track your menstrual cycle to identify your LMP and predict future periods.
BMI Calculator Philippines — Your pre-pregnancy BMI affects recommended weight gain during pregnancy. Check yours here.
TDEE Calculator — Your calorie needs increase during pregnancy. Find your baseline daily energy expenditure.