What a Pregnancy Calculator Does
A Pregnancy Calculator estimates key dates in pregnancy so you can plan with more confidence. The most common output is an estimated due date, also called an estimated due date (EDD). From there, a good pregnancy due date calculator also shows your gestational age (how many weeks and days pregnant you are), which trimester you’re in, and how many days remain until the estimate. Many people also want a week-by-week pregnancy timeline because appointments, screening windows, travel planning, and trimester transitions often revolve around “how many weeks” you are.
This tool supports multiple ways to calculate pregnancy dates because not everyone has the same information. Some people know the first day of the last menstrual period (LMP). Others may not know LMP accurately, especially with irregular cycles, recent birth control changes, or postpartum cycles. Some people know a conception date based on ovulation tracking. IVF pregnancies often have even more precise dating information because embryo age at transfer is documented. Each method can produce a useful estimate, but the most consistent timeline is the one that matches how your care team dates the pregnancy.
Gestational Age vs Time Since Conception
A common source of confusion is the difference between gestational age and the time since conception. Gestational age is the standard used in prenatal care, and it is counted from LMP. Because conception typically occurs about two weeks after LMP in a 28-day cycle, gestational age is often about two weeks more than the time since conception. For example, “8 weeks pregnant” usually means roughly 6 weeks since conception. This is why pregnancy is described as 40 weeks from LMP even though conception-to-birth is closer to 38 weeks.
A target date can still be very helpful even though it’s not exact. The purpose of a due date estimate is to provide an organized timeline: trimester dates, typical screening windows, and a way to describe progress consistently. If you have an ultrasound-based due date or an IVF-based due date, that date often becomes the best anchor for planning week-by-week.
How Due Date Is Estimated from LMP
The most common method adds 280 days (40 weeks) to the first day of your last menstrual period. This is a practical model that works reasonably well for many pregnancies, but it assumes a typical cycle length. If your cycle is longer than 28 days, ovulation often occurs later, which can shift timing. If your cycle is shorter, ovulation often occurs earlier, which can shift timing the other way. This pregnancy calculator includes a cycle length adjustment so you can refine the estimate when your average cycle differs from 28 days.
It’s also normal for cycle length to vary. If you do not have a stable average, treat the result as a planning estimate rather than an exact prediction. If your clinician dates the pregnancy by ultrasound, use that due date for the most consistent timeline.
How Due Date Is Estimated from Conception Date
If you know a likely conception date, one common approach is to add 266 days (38 weeks). This aligns with the idea that conception occurs about 14 days after LMP in a typical cycle, and 280 days from LMP minus about 14 days equals 266 days from conception. A conception-based due date can be helpful for people who track ovulation, use fertility monitors, or have an estimated date based on timed intercourse.
However, conception timing can still be uncertain because sperm can survive for several days and ovulation can vary. If your conception date is an approximation, your due date will also be an approximation. The value is still real: it gives you a structured view of weeks and months so you can plan.
How Due Date Is Estimated for IVF Transfer
IVF dating can be more precise because embryo age at transfer is known. A practical estimate is to add 261 days for a day-5 embryo transfer or 263 days for a day-3 transfer. This converts embryo age and transfer date into an implied conception timing and then into the standard 40-week pregnancy framework. If your clinic provides a specific due date, use that date as your anchor because it will match your medical chart and appointment schedule.
Trimester Dates and Why They Matter
Trimesters are a convenient way to describe stages of pregnancy. People often notice major shifts in energy, nausea, appetite, and comfort as pregnancy progresses. Many resources describe the first trimester through 13 weeks 6 days, the second trimester from 14 weeks to 27 weeks 6 days, and the third trimester from 28 weeks onward. Your prenatal care schedule also changes by trimester, with more frequent visits later in pregnancy.
A trimester timeline is helpful for planning, but remember that symptoms and experiences vary. Some people feel better early, others later, and many have a mix. The best use of trimester dates is to improve organization: appointments, travel windows, workload planning, and conversations with your support network.
Milestones and Common Test Windows
Many people want a “milestone planner” for pregnancy. Milestones can include trimester transitions, the end of the first trimester, viability milestones, and the “full-term” window. There are also common prenatal screening windows, which can differ by region and medical guidance. This calculator’s Milestones tab provides a planning view of typical windows so you can ask informed questions and schedule time appropriately.
Always follow the schedule provided by your clinician or maternity provider. Individual recommendations depend on health history, pregnancy risk factors, and local guidelines. The goal of a milestone table is to help you see the timeline clearly, not to replace professional care.
Week-by-Week Pregnancy Timeline for Planning
A week-by-week table turns your due date into a calendar you can use. It makes it easier to plan around work obligations, travel, family events, and appointment timing. Many people track their pregnancy with “weeks and days,” because prenatal care and fetal development references often use that format. The Week-by-Week tab builds a clear schedule from your anchor date and highlights your current week as of the date you choose.
If your due date changes after ultrasound dating, rebuild the timeline using the updated due date. This keeps everything consistent: week count, trimester boundaries, and milestone windows.
How to Use This Pregnancy Calculator
Use the LMP tab if you know the first day of your last period and your cycles are reasonably regular. If your cycle length differs from 28 days, enter the average and the calculator will adjust the estimate. Use the Conception / IVF tab if you have a conception date estimate or an IVF transfer date. Once you calculate, use the Milestones and Week-by-Week tabs with “Use last calculated dates” to generate a full timeline without re-entering everything.
If you prefer to anchor everything to a due date you were given, go to Milestones or Week-by-Week, select “Enter Due Date,” and build your tables from that date. This is often the most practical approach when you want your planning calendar to match your medical chart.
Important Limitations and Safety Notes
This pregnancy calculator is for education and planning. It does not provide medical advice. If you have bleeding, pain, dizziness, or concerning symptoms, contact a qualified healthcare professional. If you have irregular cycles, recent hormonal changes, or are unsure of dates, ultrasound dating may provide a more accurate due date. Always use the due date and schedule recommended by your clinician for medical decisions and appointment timing.
FAQ
Pregnancy Calculator – Frequently Asked Questions
Quick answers about due dates, gestational age, trimester timing, and using LMP, conception, or IVF transfer dates.
Most due date estimates start from the first day of your last menstrual period (LMP) and add 280 days (40 weeks). If your cycle is longer or shorter than 28 days, the estimate can be adjusted.
Gestational age is the age of the pregnancy measured from the first day of the last menstrual period (LMP). It is usually about two weeks more than the time since conception.
No. A due date is an estimate. Many healthy pregnancies deliver before or after the estimated date, and timing can vary based on individual factors and how the pregnancy is dated.
You can estimate from a known conception date, IVF transfer date, or a clinician’s ultrasound dating. This calculator supports multiple methods to provide a reasonable estimate.
A common approach is to add 266 days (38 weeks) to the conception date. This corresponds to 280 days (40 weeks) from LMP, which is about two weeks earlier than conception.
A practical estimate is to add 261 days for a 5-day embryo transfer or 263 days for a 3-day embryo transfer, then track gestational age from the implied LMP.
Many references define the first trimester up to 13 weeks 6 days, the second trimester from 14 weeks to 27 weeks 6 days, and the third trimester starting at 28 weeks.
EDD stands for Estimated Due Date. It is the same concept as due date: a best estimate based on LMP, conception/IVF dates, or ultrasound dating.
No. This tool is for planning and education. For medical decisions and the most accurate dating, follow your clinician’s guidance and ultrasound results.