Congratulations — if you are here, chances are you just found out some exciting news! One of the first questions every expecting parent asks is "when is my baby due?" A pregnancy due date calculator gives you an answer in seconds, based on the first day of your last period.
Keep in mind that only about 5% of babies arrive on their exact due date — most come within a two-week window around it. This guide walks you through how due dates are calculated, what happens in each trimester, a fun week-by-week size comparison (your baby goes from a raspberry to a watermelon!), and the prenatal care milestones you should not miss.
Calculate Due Date — Free, Private
Due date, week tracker, trimester guide.
How to Calculate Your Due Date
- Open: Go to ToolsArena Pregnancy Calculator
- Enter LMP: First day of your last menstrual period
- Get results: Due date, current week, trimester, countdown
The Formula (Naegele's Rule)
Due Date = LMP + 280 days (40 weeks). Only 5% of babies arrive on the exact due date.
Trimester Guide
| Trimester | Weeks | Key Developments |
|---|---|---|
| First | 1-12 | Heart beats (week 6), organs form, morning sickness |
| Second | 13-26 | Baby kicks (week 18-20), gender visible, energy returns |
| Third | 27-40 | Rapid weight gain, lungs mature, nesting instinct |
Week-by-Week Baby Size
| Week | Size | Like |
|---|---|---|
| 8 | 1.6 cm | Raspberry |
| 12 | 5.4 cm | Lime |
| 16 | 11.6 cm | Avocado |
| 20 | 16.5 cm | Banana |
| 24 | 30 cm | Corn |
| 28 | 37.6 cm | Eggplant |
| 32 | 42.4 cm | Squash |
| 36 | 47.4 cm | Papaya |
| 40 | 51.2 cm | Watermelon |
Prenatal Care Tips
- First trimester: Start prenatal vitamins (folic acid), schedule OB appointment, avoid alcohol
- Second trimester: Anatomy ultrasound (week 18-20), glucose screening
- Third trimester: Hospital bag by week 36, birth plan ready
Disclaimer: Estimates only. Consult your healthcare provider.
Due Date Calculation Methods
| Method | Based On | Accuracy |
|---|---|---|
| LMP | Last period | Most common |
| Ultrasound | Fetal measurements | Most accurate |
| Conception date | Known ovulation | Very accurate |
| IVF transfer | Embryo transfer | Most precise |
How to Use the Tool (Step by Step)
- 1
Open Pregnancy Calculator
Free, private.
- 2
Enter last period date
First day of LMP.
- 3
Get due date
Due date, week, trimester.
- 4
Track milestones
Week-by-week development.
Frequently Asked Questions
How is the due date calculated?+−
The standard method (called Naegele Rule) adds 280 days (40 weeks) to the first day of your last menstrual period. Your doctor uses the same formula, though they may adjust it after your first ultrasound if the baby measures differently than expected.
How accurate is a due date calculator?+−
Think of it as an estimate, not a deadline. Only about 5% of babies arrive on the exact due date. Most come within a two-week window — so if your due date is March 15, expect baby anytime between March 1 and March 29. Your doctor will give you a more precise date after ultrasound measurements.
How many weeks is a full pregnancy?+−
40 weeks from the first day of your last period — that is about 9 months and 1 week. Fun fact: pregnancy is actually counted from before conception, since ovulation typically happens around week 2.
What if my cycle is not exactly 28 days?+−
If your cycle is longer (say 35 days), your due date shifts later because you likely ovulated later. If it is shorter (say 25 days), the due date moves earlier. Tell your doctor your average cycle length — they will adjust the calculation.
When should I schedule my first doctor visit?+−
Most OB-GYNs recommend scheduling your first prenatal visit around week 8-10, or as soon as you get a positive test. They will confirm the pregnancy, estimate the due date with an ultrasound, and start your prenatal care plan.
What trimester am I in?+−
First trimester: weeks 1-12 (early development, morning sickness). Second trimester: weeks 13-26 (baby kicks, gender visible, energy returns). Third trimester: weeks 27-40 (rapid growth, preparation for birth). The calculator shows your current trimester automatically.
Is this calculator a substitute for medical advice?+−
Absolutely not — it provides estimates for informational purposes only. Always work with your OB-GYN or midwife for medical decisions. Every pregnancy is unique, and your healthcare provider knows your specific situation.
Can I calculate from the conception date instead?+−
Yes — if you know exactly when you conceived (common with IVF or tracked ovulation), the due date is conception date + 266 days. This is actually more accurate than the LMP method since it removes the guesswork about when ovulation happened.
Calculate Due Date — Free, Private
Due date, week tracker, trimester guide.
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