Pregnancy Calculator
Enter the first day of your last menstrual period or the due date, and you'll see the pregnancy week, trimester, and important dates.
Key milestones
Pregnancy Calculator — Due Date and Weekly Tracker
The pregnancy calculator estimates your due date, current pregnancy week, and trimester based on the first day of your last menstrual period (LMP). It also highlights important milestone dates such as ultrasound windows and the start of each trimester.
How Are Pregnancy Weeks Calculated?
Pregnancy is dated from the first day of the last menstrual period (LMP), not from the date of conception. Because ovulation typically occurs about two weeks after LMP, the first two "weeks" of pregnancy actually occur before the egg is fertilized. The estimated due date (EDD) is 40 weeks (280 days) from the LMP.
Due Date = LMP + 280 days
This method is known as Naegele's Rule, named after the German obstetrician who popularized it in the early 1800s.
Pregnancy Trimesters
| Trimester | Weeks | Key Developments |
|---|---|---|
| 1st trimester | Weeks 1 – 12 | Implantation, heartbeat begins (week 6), morning sickness common, all major organs form |
| 2nd trimester | Weeks 13 – 27 | Baby kicks felt (weeks 18–22), gender visible on ultrasound, rapid growth |
| 3rd trimester | Weeks 28 – 40 | Lungs mature, baby gains weight, nesting instinct, preparation for birth |
Week-by-Week Size Comparison
| Week | Baby Size | Comparable To |
|---|---|---|
| 8 | 0.6 in (1.6 cm) | Raspberry |
| 12 | 2.1 in (5.4 cm) | Lime |
| 16 | 4.6 in (11.6 cm) | Avocado |
| 20 | 6.5 in (16.4 cm) | Banana |
| 24 | 11.8 in (30 cm) | Ear of corn |
| 28 | 14.8 in (37.6 cm) | Eggplant |
| 32 | 16.7 in (42.4 cm) | Squash |
| 36 | 18.7 in (47.4 cm) | Honeydew melon |
| 40 | 20.2 in (51.2 cm) | Watermelon |
Important Prenatal Appointments
- First prenatal visit (weeks 8–10) — Confirms pregnancy, estimates due date, orders blood work
- Dating ultrasound (weeks 11–14) — Measures fetal size, checks nuchal translucency for chromosomal screening
- Anatomy scan (weeks 18–22) — Detailed check of organs, spine, brain; gender may be revealed
- Glucose screening (weeks 24–28) — Tests for gestational diabetes
- Group B strep test (weeks 35–37) — Determines if antibiotics are needed during labor
- Weekly visits (weeks 36–40) — Monitoring position, heartbeat, cervical changes
How Accurate Is the Due Date?
The due date is an estimate, not a deadline. Full-term birth spans weeks 37 through 42. Statistically:
- Only about 5% of babies are born on the exact due date
- 80% are born within 2 weeks of the due date (38–42 weeks)
- First-time mothers tend to deliver slightly later than the EDD
An early ultrasound (before 14 weeks) is the most accurate method for confirming the due date, often more precise than LMP-based calculation.
Frequently Asked Questions
How do I calculate my due date without knowing my LMP?
If your periods are irregular, a dating ultrasound in the first trimester is the best way to estimate gestational age. The ultrasound measures the crown-rump length of the fetus.
Can my due date change?
Yes. If an early ultrasound shows a significant discrepancy from the LMP-based date, your provider may adjust the due date. Later ultrasounds are less accurate for dating.
What is the difference between gestational age and fetal age?
Gestational age counts from LMP (includes ~2 weeks before conception). Fetal age (embryonic age) counts from actual conception and is about 2 weeks less.
Is it safe to exercise during pregnancy?
In most healthy pregnancies, moderate exercise is recommended. The ACOG suggests at least 150 minutes of moderate activity per week. Always consult your healthcare provider.
Related Tools
- Age Calculator — Calculate exact age in years, months, and days
- Date Calculator — Find the difference between any two dates
- Sleep Cycle Calculator — Optimize your sleep during pregnancy
- Daily Calorie Needs Calculator — Adjust calorie intake for pregnancy
- BMI Calculator — Check pre-pregnancy BMI
Sources
- Mayo Clinic: Pregnancy Week by Week
- ACOG: How Your Fetus Grows During Pregnancy
- NIH: What is Prenatal Care?