Educational tool only. Always confirm dates and care decisions with your OB/GYN or midwife.
What this gestation weeks calculator does
This calculator estimates how far along a pregnancy is in weeks and days. It also gives a projected due date, a trimester estimate, and how many days remain until delivery. In most obstetric settings, gestational age is counted from the first day of the last menstrual period (LMP), even though conception usually happens about two weeks later.
If you do not know your LMP, you can estimate gestation from conception date or estimated due date instead. These methods are useful when cycles are irregular or when your care provider has already given you a due date based on ultrasound findings.
How to use the calculator
- Select your method: LMP, conception date, or due date.
- Enter the relevant date field.
- If using LMP, adjust cycle length if your cycle is usually longer or shorter than 28 days.
- Set an “as of” date (defaults to today).
- Click Calculate to view gestational age and timeline details.
Understanding your result
Gestational age
Displayed as weeks + days (for example, 22 weeks 3 days). This is the standard format used in prenatal care and ultrasound reports.
Estimated due date (EDD)
The EDD is commonly set at 40 weeks from LMP (280 days), or 266 days from conception. It is an estimate, not a guarantee. Most births happen in a range around that date.
Trimester
- First trimester: 0 to 13 weeks + 6 days
- Second trimester: 14 to 27 weeks + 6 days
- Third trimester: 28 weeks onward
Why dates can change during pregnancy
A due date can shift after early ultrasound, especially when menstrual dates are uncertain or cycles are irregular. Clinicians may revise the “best obstetric estimate” to match fetal measurements in the first trimester.
Common reasons for date adjustments include:
- Uncertain LMP recall
- Irregular menstrual cycles
- Recent hormonal contraception use
- Ovulation occurring earlier or later than average
- Differences between menstrual dating and ultrasound dating
Typical milestone weeks
- 8-12 weeks: early prenatal screening and heartbeat confirmation
- 18-22 weeks: anatomy ultrasound
- 24-28 weeks: glucose screening for gestational diabetes
- 35-37 weeks: Group B strep testing window
- 37+ weeks: considered early term and approaching full term birth window
FAQ
Is gestational age the same as fetal age?
Not exactly. Gestational age starts at LMP and is usually about two weeks ahead of fetal age (time since conception).
Can this replace an ultrasound?
No. This tool provides an estimate based on your dates. Ultrasound and professional prenatal assessment are the standard for medical decisions.
What if I am past 40 weeks?
Many healthy pregnancies continue beyond 40 weeks. Follow your provider’s monitoring plan and labor guidance.
Important note
This page is for education and planning only. If you have pain, bleeding, decreased fetal movement, severe headache, swelling, fever, or any urgent concern, contact your maternity care team immediately.