Pregnancy Countdown Calculator
Enter your estimated due date to see exactly how many weeks and days remain, which trimester you are in, how many days along you are, and your approximate conception date. Key milestones from week 8 to full term are plotted on a live timeline so you can see the whole journey at a glance.
How due dates are calculated
The standard method used by healthcare providers worldwide is Naegele's Rule: take the first day of your last menstrual period (LMP) and add 280 days, or equivalently 40 weeks. This date is called the estimated due date (EDD) or estimated delivery date. The rule works because it counts backwards from when fertilization actually occurred - ovulation typically happens around day 14 of a 28-day cycle, making the total time from conception to birth approximately 266 days (38 weeks). Adding those 14 days to align with the LMP gives the 280-day figure. Your due date may also be set or adjusted by an early ultrasound, which measures the baby's size (crown-to-rump length) and is most accurate between weeks 7 and 12.
Understanding the three trimesters
Pregnancy is divided into three trimesters, each roughly three months long. The first trimester (weeks 1-12) covers the most critical period of development: the embryo implants, all major organs begin to form, and the heartbeat becomes visible on ultrasound from around week 6. The second trimester (weeks 13-27) is often described as the most comfortable: morning sickness usually eases, energy returns, and parents-to-be can often feel the baby move for the first time between weeks 18 and 22. The anatomy scan at around week 20 checks organ development in detail. The third trimester (weeks 28-40) is the final preparation: the baby gains most of its weight, the lungs mature, and the body prepares for birth. Full term is defined as weeks 39-40, but babies born from 37 weeks onwards are considered early term. A pregnancy lasting beyond 42 weeks is classified as post-term.
Why only about 5% of babies arrive on the due date
The EDD is the midpoint of a statistical distribution, not a firm deadline. Research published by the CDC shows that roughly 56% of births occur in the week of or the week before the due date. The remaining 44% arrive either earlier or later. Natural variation in ovulation timing, implantation, and individual physiology all contribute to this spread. A range of 37 to 42 completed weeks is considered normal. Your healthcare provider will monitor the pregnancy closely if it extends beyond 41 weeks and may recommend induction of labor after 41-42 weeks, depending on clinical guidelines and individual circumstances.
Key milestones to look forward to
Knowing which milestones are coming up makes the countdown more meaningful. At week 4, many people discover they are pregnant when a period is missed. At week 6-8, the heartbeat is typically visible on an early scan. Week 12 marks the end of the first trimester, when the risk of miscarriage drops significantly and many couples share their news. Week 20 brings the detailed anatomy scan. Around weeks 18-22, fetal movement (quickening) is usually felt for the first time. Week 28 marks the start of the third trimester, after which a baby born prematurely has an increasingly high chance of survival with neonatal care. Weeks 36-38 bring the baby's head down into position (engagement or "lightening") in preparation for birth.
Pregnancy trimesters at a glance
| Trimester | Weeks | Days from LMP | Key developments |
|---|---|---|---|
| 1st trimester | Weeks 1-12 | Days 1-84 | Implantation, organ formation, heartbeat (wk 6), risk of miscarriage highest |
| 2nd trimester | Weeks 13-27 | Days 85-189 | Movement felt, anatomy scan (wk 20), sex may be determined, energy returns |
| 3rd trimester | Weeks 28-40 | Days 190-280 | Rapid weight gain, lung maturation, Braxton-Hicks, birth preparation |
Standard clinical trimester divisions for a 40-week (280-day) pregnancy.
Frequently asked questions
How accurate is the due date?
The estimated due date (EDD) is an approximation, not a guarantee. Only about 5% of babies are born on their exact due date, and roughly 56% arrive within a week of it. Early ultrasound dating (before 12 weeks) is the most accurate method and can refine the EDD if there is a discrepancy of more than 5-7 days from the LMP-based date. Natural variation in ovulation timing means the LMP method alone has an inherent spread of about 1-2 weeks.
What is the difference between gestational age and fetal age?
Gestational age is counted from the first day of the last menstrual period (LMP) and is the standard clinical measure. It includes the roughly two weeks before conception actually occurred. Fetal age (or embryonic age) is counted from fertilization and is therefore about two weeks shorter. When your healthcare provider says you are "12 weeks pregnant," they almost always mean 12 weeks gestational age from LMP.
What if I do not know my last period date?
If you are unsure of your LMP, use the due date (EDD) input mode and enter the date given to you by your doctor or from your ultrasound report. An early ultrasound between 7 and 12 weeks can accurately determine gestational age from the baby's crown-to-rump length and is considered the most reliable dating method when LMP is uncertain.
When does each trimester start?
The first trimester runs from week 1 (LMP day 1) through the end of week 12 (LMP day 84). The second trimester begins at week 13 (LMP day 85) and ends at the close of week 27 (LMP day 189). The third trimester starts at week 28 (LMP day 190) and continues until birth, with 40 weeks (LMP day 280) being the standard estimated due date.
What is "full term" and when is it safe to deliver?
The American College of Obstetricians and Gynecologists (ACOG) defines full term as 39 to 40 weeks and 6 days. Early term is 37 to 38 weeks and 6 days. Late term is 41 to 41 weeks and 6 days. Post-term is 42 weeks or beyond. While babies born at 37 weeks are generally healthy, weeks 39 and 40 are associated with the best outcomes for lung maturation, feeding, and neurological development.
How is the conception date calculated?
This calculator estimates the conception date as LMP plus 14 days, which corresponds to day 14 of a standard 28-day cycle - when ovulation typically occurs. This is an approximation: ovulation can happen anywhere from day 10 to day 21 depending on individual cycle length, and sperm can survive for up to 5 days in the reproductive tract. For most purposes, the conception date is an interesting landmark rather than a clinically precise figure.