Pregnancy Trimester Dates Calculator
The Pregnancy Trimester Dates calculator shows trimester start and end dates and the current gestational week using either LMP or a known due date.
This calculator lets us input the last menstrual period or an estimated due date, then instantly maps out trimester boundaries in MM-DD-YYYY. The goal is a simple, shareable schedule for planning visits and milestones. If we’d rather do it by hand, we include the timelines and formulas below.
Enter either the first day of the last menstrual period (LMP) or an estimated due date (EDD). We compute trimester boundaries and your current week. Dates display in MM-DD-YYYY. For medical questions, please talk to your clinician.
We’ll convert to trimester dates automatically.
If you have an EDD from ultrasound or your provider, enter it here.
Please enter a valid LMP in MM-DD-YYYY.
Results interpretation
How it works
Gestational age is traditionally measured from the first day of the last menstrual period (LMP). A full-term pregnancy is assumed to be 280 days (40 weeks).
Formulas, assumptions, limitations
From LMP to EDD. EDD = LMP + 280 days. We add whole days to avoid time-zone drift.
Trimester boundaries. First: 0–13 weeks; Second: 14–27 weeks; Third: 28–40 weeks. We show start and end dates for each block.
Current week. We compute the difference between today and LMP in days, then convert to weeks + days.
When EDD is known. If you enter EDD, we infer LMP = EDD − 280 days so that boundaries line up with your due date.
Important note. Clinical dating can change after ultrasound. Treat this as a planning aid, not medical advice.
Use cases & examples
EDD ≈ 10-17-2025; First trimester ends 04-18-2025; Second trimester spans 04-19-2025 to 07-25-2025; Third trimester starts 07-26-2025 and ends on EDD.
LMP ≈ 03-06-2025; Boundaries are computed from that LMP and align with the due date you provided.
If the LMP is in the future (e.g., planning), we’ll show “Before timeline” and Week 0 + 0 d until the LMP date arrives.
Pregnancy Trimester Dates FAQs
Why do my dates differ from my provider’s?
Clinicians may adjust dates based on ultrasound. Our tool uses a standard 280-day model and a common trimester definition.
Can I enter both LMP and due date?
Use one at a time. If you have a provider-given due date, pick “Use Due Date (EDD)” for the closest alignment.
What date format do you use?
We display MM-DD-YYYY across the app for consistency.
Is this medical advice?
No. This is an educational tool. Always follow guidance from your clinician.
How trimester dating works
Pregnancy timing has two common anchors: the first day of the last menstrual period (LMP) and the estimated due date (EDD). In a traditional 40-week model, the due date is 280 days after LMP. Because ovulation typically occurs about two weeks after LMP in a 28-day cycle, gestational age counts two weeks before conception. This convention keeps charts, research, and care plans consistent across different cycles.
Why boundaries vary by source
Some references define the first trimester as 0–12 weeks and others as 0–13 weeks. Many clinical schedules use 0–13, 14–27, and 28–40. We adopt this widely used split so visit timing and screening windows feel familiar. If your provider uses a slightly different boundary, follow their guidance—the differences are a matter of convention, not a change in your baby’s development.
Using LMP vs. using a due date
If cycles are regular and you tracked your LMP, using LMP is straightforward. If your cycles are irregular or if an early ultrasound provided a due date, entering EDD often better reflects your clinical plan. Our calculator supports both: with EDD, we infer the matching LMP so trimester boundaries still line up with your due date.
Current week, explained
The “current week” label shows how far along you are today. For example, if 21 weeks and 3 days have elapsed since LMP, we present Week 21 + 3 d. The number updates automatically when you reload the page on a later date.
Planning around milestones
Knowing trimester dates helps schedule visits, screening tests, classes, and personal plans. It can also make communication with family and work easier—everyone’s working from the same calendar.
Limitations of calendar-only timelines
Babies have their own timelines. Ultrasound measurements, health considerations, and multiple gestations (twins or more) may lead your clinician to adjust dates. Think of our tool as a clear, consistent baseline to organize life around—not a guarantee of specific delivery timing.
Tips for accurate entries
- Use official dates from your medical portal when available.
- Enter the first day of bleeding for LMP (not spotting afterward).
- Re-check your EDD after an early ultrasound; providers commonly adjust it then.
- Share your printed or saved schedule with your support team.