👶 Pregnancy Due Date Calculator
Calculate estimated due date and current gestational age based on your last menstrual period (LMP).
Gestational age: how far along you are, and what that means
Gestational age is counted from the first day of your last menstrual period, not from conception. That convention adds roughly two weeks at the front during which you were not pregnant, which is why a pregnancy is described as forty weeks when the embryo is only about thirty-eight weeks old. It exists because the date of the last period is something most people know and the date of conception usually is not.
This calculator tells you how many weeks and days along you are today, which trimester that falls in, and what is typically happening at that stage. If you want the estimated delivery date itself, the due date calculator is the one to use.
The three trimesters
| Trimester | Weeks | What is typically happening |
|---|---|---|
| First | 0 – 13 | Organ formation, highest miscarriage risk, nausea common from week 6 |
| Second | 14 – 27 | Nausea usually eases, movement first felt around 18–22 weeks, anatomy scan at 18–22 |
| Third | 28 – 40+ | Rapid growth, viability improving each week, monitoring increases after 36 |
Boundaries vary slightly between health systems: some put the first trimester at 0–12 weeks and the second at 13–27. The difference is a labelling convention and does not change any clinical decision.
Term definitions
Since 2013 the American College of Obstetricians and Gynecologists has used four categories instead of a single idea of term, because outcomes differ measurably across them:
| Category | Weeks | |
|---|---|---|
| Preterm | before 37 weeks 0 days | Increased need for neonatal support |
| Early term | 37 weeks 0 days – 38 weeks 6 days | Slightly higher risk than full term |
| Full term | 39 weeks 0 days – 40 weeks 6 days | Best average outcomes |
| Late term | 41 weeks 0 days – 41 weeks 6 days | Monitoring usually increases |
| Post term | 42 weeks 0 days and beyond | Induction normally discussed before this |
Why the calculated date is only an estimate
The counting method assumes a 28-day cycle with ovulation on day 14. Cycles vary from 21 to 35 days in healthy adults, and ovulation timing varies even within one person's cycles. If your cycle averages 32 days you probably ovulated around day 18, so your true gestational age is about four days behind what the standard count says.
This is why a first-trimester dating ultrasound takes precedence. Measuring crown-rump length between 8 and 13 weeks estimates gestational age within about five days, which is considerably more accurate than any date-based calculation. If the scan and the LMP disagree by more than about a week, clinicians normally redate the pregnancy to the scan.
Only around 4% of babies arrive on their estimated due date, and roughly 80% arrive within the two weeks either side of it. The date is the centre of a distribution, not an appointment.
Working out your own dates
To find gestational age by hand, count the days from the first day of your last period to today, then divide by seven. A remainder gives the days: 164 days is 23 weeks and 3 days. To adjust for a longer cycle, subtract the difference from 28 before counting; for a 32-day cycle, subtract four days of gestational age.
A worked example
Last period started 3 March. Today is 25 October, which is 236 days later: 33 weeks and 5 days, firmly in the third trimester. The estimated due date, 280 days from 3 March, is 8 December. With a 31-day cycle instead of 28, the true gestational age would be closer to 33 weeks and 2 days, and the estimated date would shift to 11 December.
Every pregnancy is different and these dates are estimates based on population averages. Your midwife, obstetrician or doctor, with an ultrasound, is the authority on your dates. Nothing on this page is medical advice.
Frequently Asked Questions (FAQs)
Why is pregnancy counted from my last period rather than conception?
Because the date of the last period is reliably known and the date of conception usually is not. The convention adds about two weeks at the start, so gestational age is roughly two weeks ahead of the embryo's actual age. All clinical milestones and charts use this same convention.
How accurate is a dating ultrasound?
A first-trimester scan measuring crown-rump length between 8 and 13 weeks is accurate to within about five days. Accuracy falls as pregnancy progresses because growth varies more between babies, so a third-trimester scan can be out by two to three weeks and is not used for dating.
What if my cycle is not 28 days?
Standard counting assumes ovulation on day 14. With a 35-day cycle you likely ovulated around day 21, so your true gestational age is about a week behind the standard count. Enter your actual cycle length if the calculator asks for it, and expect the dating scan to be the final word.
When will I feel the baby move?
First-time mothers usually notice movement between 18 and 22 weeks, and those who have been pregnant before often notice it earlier, from around 16 weeks, because they recognise the sensation. Anterior placenta placement can delay it by a couple of weeks.
Is 37 weeks full term?
Not under current definitions. Since 2013, 37 to 38 weeks and 6 days is classed as early term, and full term starts at 39 weeks. The change was made because outcomes at 37 and 38 weeks are measurably different from those at 39 and 40.
How many weeks is nine months?
Nine calendar months is about 39 weeks, which is why the forty-week count and the nine-month description never quite line up. Forty weeks is closer to nine months and one week.