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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.

If you’re wondering if you’re pregnant, take our pregnancy quiz here.

Traditional Dating

Generally, you can tell how far along you are by the date your last period started, called your last menstrual period, or LMP.

Certain kinds of birth control may stop or cause your period to come every few months. Because of this, gestational dating based on your last period can be inaccurate. 

Nearly 20% of women were not able to accurately say when their last period was (LMP). This is why an ultrasound can be a helpful tool to determine gestational age.

If you do know when your last period was, you can calculate your estimated due date (EDD).

determining due date

Due Date Calculator

According to Johns Hopkins, you can follow these steps to estimate your due date (EDD) based on your period:

  1. Take the first date of your last period and go back three months (ex. July 19, 2023, walk back to April 19, 2023)
  2. Add one year and 7 days to your previous calculation (April 19, 2023, becomes April 26, 2024)

So, if your last period was July 19, 2023, your predicted due date is April 26, 2024.

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.

Because of the variability, doctors rely on your last menstrual period or an early ultrasound to estimate your due date.

Viability Ultrasound

Viability means “likely to live” and is determined in early pregnancy through ultrasound.

To check viability, doctors, sonographers, or trained nurses can perform an ultrasound during the first trimester. This ultrasound helps determine if the pregnancy is likely to progress successfully.

During the 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. Your nurse will also check the location of the pregnancy to rule out  ectopic pregnancy.

Size measurement, along with the presence of a heartbeat and determination of an intrauterine pregnancy (ie not ectopic) will help determine viability and how far along you are in your pregnancy.

Alpha Center provides viability 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 gestation of your pregnancy is important.

According to the FDA, Mifepristone is approved for use for the first 70 days of pregnancy. A viability 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 if this is the case for you, you will not need an abortion. Alpha Center will refer you to an OBGYN for further care.


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