One of the first questions asked after you find out that you’re pregnant is “how far along am I?” or “When is the due date?” No matter what options you may be considering for your pregnancy, answering the question of gestational age is important for planning and determining your next steps.
If you’re wondering if you’re pregnant, take our pregnancy quiz here.
Traditional pregnancy Dating
Generally, you can tell how far along you are by the date your last period started, called your last menstrual period, or LMP. However, as with most things in life, this is not the case for all women. The American College of Obstetrics and Gynecology states that generally, EDD is 280 days after the patient’s LMP.
Due Date Calculator
If you know when your last period was, you can calculate your estimated due date (EDD). According to Johns Hopkins, you can follow these steps to estimate your due date (EDD) based on your period:
- Take the first date of your last period and go back three months (ex. April 19, 2025, walk back to January 19, 2025)
- Add one year and 7 days to your previous calculation (January 19, 2025, becomes January 26, 2026)
So, if your last period was April 19, 2025, your predicted due date is January 26, 2026.
Can I calculate Due Date Based on Conception?
Determining the exact moment of conception can be quite tricky, especially if you haven’t been actively tracking your ovulation or undergoing any tests. In addition, sperm can live inside your body for up to a week after you have sex, meaning you can conceive at any point during that time.
Due to the variability of conception, doctors typically rely on your last menstrual period or an early ultrasound to estimate your due date.
What If I’m Not Sure When My Last Period Was?
If you’re not sure when your last period was, you’re not alone. Certain kinds of birth control may stop or cause your period to come every few months, and 14-25% of women experience menstrual irregularities. Additionally, nearly 20% of women were not able to accurately say when their last period was (LMP).
If you’ve received a positive pregnancy test but aren’t sure when your last period was (LMP), you can determine how far along you are by getting an ultrasound.
Gestational dating Ultrasound
To determine dating in early pregnancy, doctors, sonographers, or trained nurses can perform an ultrasound. During an ultrasound, a nurse will use an ultrasound wand (either inserted vaginally or on top of the abdomen depending on estimated gestational age), called a transducer. The transducer gives off sound waves that bounce off internal organs to create a picture of the inside of your body on the ultrasound screen. Transvaginal ultrasound is able to give your nurse a clearer and larger image for first trimester pregnancy.
Using the ultrasound image, your nurse will measure the size of the pregnancy and compare it to established growth charts to determine the gestational age of the pregnancy. If there is a discrepancy of 5 days or more between the estimated due date based on LMP and ultrasound, your provider may elect to adjust the due date.
Additional Reasons for Ultrasound
In addition to gestational age, during an ultrasound at Alpha Center, your nurse will also check the location of the pregnancy to rule out ectopic pregnancy. They will also check for the presence of a heartbeat, and if it’s a follow-up ultrasound, growth between the two scans to determine viability.
Alpha Center provides viability and gestational dating ultrasound as part of its pregnancy testing and pre-abortion screening appointments. Request an appointment today!
Do I Need to Know My Due Date if I’m Planning to Take the Abortion Pill?
Even if you are planning to take the abortion pill (aka RU486, medication abortion, or Plan C), knowing the gestational age of your pregnancy is important.
According to the FDA, Mifepristone is approved for use for the first 70 days of pregnancy. A gestational dating ultrasound can tell you if a medication abortion is an option for you, or if you don’t need an abortion because your pregnancy isn’t viable. Roughly 1/3 of pregnancies are not viable and will end in miscarriage. If our nurses suspect that you are having a miscarriage or our medical director (OBGYN) sees something that warrants follow-up on your ultrasound, Alpha Center will refer you to an OBGYN for further care.
Authored by Kristen Long for Alpha Center. Originally published December 2023. Updated July 2025.
Read More:
- Frequently Asked Questions: RU486
- What is a Pre-Abortion Screening?
- Ectopic Pregnancy: What You Need To Know
Sources:
American College of Obstetrics and Gynecology. Opinion 700. Methods for Estimating the Due Date. May 2017. https://www.acog.org/-/media/project/acog/acogorg/clinical/files/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date.pdf?rev=e94ee470b78b466ca0d9e6c4ab127979&hash=5A6DE3B09B00A8E9A90DD402DC58BDA6
Johns Hopkins Medicine. Calculating a Due Date. Accessed April 8, 2025. https://www.hopkinsmedicine.org/health/wellness-and-prevention/calculating-a-due-date
National Institute Child of Health and Human Development. What Are Menstrual Irregularities? Updated January 3, 2017. Accessed April 8, 2025. https://www.nichd.nih.gov/health/topics/menstruation/conditioninfo/irregularities
Cleveland Clinic. Transvaginal Ultrasound. Updated May 23, 2022. Accessed April 8, 2025. https://my.clevelandclinic.org/health/diagnostics/4993-transvaginal-ultrasound
National Health Service. Ectopic Pregnancy. Last Reviewed August 23, 2022. Accessed April 8, 2025. https://www.nhs.uk/conditions/ectopic-pregnancy/#:~:text=An%20ectopic%20pregnancy%20is%20when,risk%20if%20the%20pregnancy%20continues.
