How the pregnancy calculator estimates your due date
This calculator determines your estimated due date and tracks your pregnancy week-by-week using five different starting points: your last menstrual period, a known due date, an ultrasound measurement, a conception date, or an IVF transfer date. It then generates a complete 42-week timeline with milestones, baby size, and trimester transitions.
Select your calculation method, enter the relevant date, and instantly see your current week, trimester, progress, due date, and the full pregnancy timeline.
How to use it
- Choose your calculation method from the dropdown (Last Period, Due Date, Ultrasound, Conception, or IVF Transfer).
- Enter the corresponding date and any additional details (cycle length, ultrasound weeks, embryo age).
- Read your current week, trimester, days remaining, and progress percentage.
- Scroll the timeline table to see milestones for every week of pregnancy.
How due date is calculated
From last menstrual period (LMP)
The standard medical calculation adds 280 days (40 weeks) to the first day of your last period. If your cycle is longer or shorter than 28 days, the calculator adjusts:
This is called Naegele's rule and is the method most healthcare providers use at your first appointment.
From a known due date
If your provider has already given you a due date (from an early ultrasound), enter it directly. The calculator works backward to determine your LMP and current week.
From ultrasound
Enter the date your ultrasound was performed and the gestational age it reported (weeks and days). The calculator determines your LMP by subtracting the gestational age from the ultrasound date. First-trimester ultrasounds (before 12 weeks) are accurate to within 5 days.
From conception date
If you know when conception occurred (tracked ovulation, timed intercourse), the calculator adds 266 days for the due date. Conception is assumed to occur 14 days after LMP in a standard cycle.
From IVF transfer date
For IVF pregnancies, the exact embryo age is known. The calculator accounts for whether a Day-3 or Day-5 (blastocyst) embryo was transferred:
IVF dates produce the most precise due date estimate since there is no uncertainty about when fertilization occurred.
Understanding your pregnancy timeline
Trimesters
Pregnancy is divided into three trimesters, each with distinct developmental stages:
- First trimester (weeks 1–12): All major organs form. Highest risk of miscarriage. Heartbeat detectable by week 6. Morning sickness peaks around weeks 8–10.
- Second trimester (weeks 13–27): Baby grows rapidly, begins moving, and can hear. Anatomy scan around week 20. Often called the most comfortable trimester.
- Third trimester (weeks 28–40+): Baby gains weight, lungs mature, and prepares for birth. Full term is reached at week 38.
Key milestones
The timeline table marks significant developmental events:
| Week | Milestone |
|---|---|
| 3 | Conception occurs |
| 4 | Pregnancy test can turn positive |
| 6 | Heartbeat detectable by ultrasound |
| 13 | Miscarriage risk drops significantly |
| 18 | Baby moves noticeably, can hear, gender visible |
| 23 | Premature baby may survive with intensive care |
| 28 | Baby can breathe (lungs produce surfactant) |
| 38 | Full term — baby is ready for birth |
Baby size by week
From about week 8 onward, the calculator shows your baby's approximate length and weight. These are averages from large population studies. Individual babies vary, and ultrasound measurements at your appointments are more accurate for your specific pregnancy.
How "weeks pregnant" is counted
Gestational age counts from the first day of your last period, not from conception. This means you are technically "2 weeks pregnant" before the egg is even fertilized (ovulation typically occurs around day 14). This convention exists because LMP is a known, observable date while conception usually is not.
Common mistakes to avoid
- Using the wrong LMP date. Enter the first day of bleeding (not the last day, not spotting). If you're unsure, use the ultrasound method instead.
- Ignoring irregular cycles. If your cycle varies significantly (more than 5 days month to month), the LMP method is less reliable. An early ultrasound or known conception date is more accurate.
- Expecting delivery on the due date. Only about 5% of babies arrive on the exact due date. The normal window is 38–42 weeks. Consider the due date a midpoint estimate, not a deadline.
- Comparing to other pregnancies. Every pregnancy progresses differently. Baby size, symptom timing, and delivery date vary widely even between pregnancies in the same person.
Limitations
This calculator provides estimates based on population averages for a single pregnancy. Twin or multiple pregnancies follow different timelines and typically deliver earlier. The baby size data represents 50th percentile averages. Your healthcare provider's measurements and assessments always take priority over calculator estimates. This tool is for informational tracking purposes, not medical diagnosis.
Frequently asked questions
How is the due date calculated from my last period?
The standard method adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). If your cycle is longer or shorter than 28 days, the calculator adjusts by adding the difference. For example, a 32-day cycle adds 4 extra days to the due date.
Which method is most accurate for estimating the due date?
First-trimester ultrasound (before 12 weeks) is generally the most accurate, within ±5 days. LMP-based calculation is within ±2 weeks for regular cycles. IVF dates are the most precise since the exact fertilization date is known. Conception date is accurate if you can pinpoint ovulation.
What does 'weeks pregnant' actually mean?
Gestational age counts from the first day of your last period, not from conception. So at 'week 3' the embryo was just conceived (ovulation is around day 14). This can be confusing — you are medically '4 weeks pregnant' when your period is just one week late.
How accurate is the due date?
Only about 5% of babies are born on their exact due date. Most are born within 2 weeks of it (between 38 and 42 weeks). The due date is the midpoint of a normal delivery window, not a deadline. First pregnancies tend to go slightly past the due date.
When do I change trimesters?
The first trimester is weeks 1–12, the second trimester is weeks 13–27, and the third trimester is weeks 28–40+. Each trimester is roughly 13 weeks. The transitions are approximate — different sources vary by a week.
Can my due date change during pregnancy?
Yes. If an early ultrasound shows the baby measuring significantly different from the LMP-based estimate (more than 5–7 days), your provider may adjust the due date. Later ultrasounds are less accurate for dating because growth varies more between babies.
What if I do not know my last period date?
Use the ultrasound mode if you have had a dating scan, or the due date mode if your provider has already given you a due date. An early ultrasound (6–12 weeks) is accurate for dating even without knowing the LMP.