When Did I Conceive Based on Ultrasound?

When Did I Conceive Based on Ultrasound?

If you find out that you’re pregnant, the first question that comes to mind is what time you’ll be due. But if yours is unexpected or unplanned, you might query the point when the pregnancy happened. You’ll often ask yourself, ‘when did I conceive based on ultrasound?’

An obstetric ultrasound exam is one of the best ways to tell the date you conceived. Using the ‘crown to rump ratio,’ the sound wave scanner helps a doctor measure the length of the embryo to offer gestational age. Subtracting two weeks then provides a conception date, which while accurate to within a few days, gives fetal age that translates to how long you’ve been pregnant.

Growth rates for babies worldwide are the same, and ultrasound gives an accurate estimate of when you conceived. Want to know that information is influenced by the need to pinpoint the baby’s father, examine your career options, or start access to prenatal care. Please keep reading to learn the workings of ultrasounds, their types, and when’s the best time to take a test that determines the conception date.

What’s an Ultrasound, and How Does It Calculate the Time of Conception?

Ultrasounds are a medical technology that uses sound waves to give a clear picture of what’s happening inside your body. While used in many applications within the medical field, these machines are often talked of concerning pregnancy. There’s no risk to you or the growing embryo, and trained technicians or sonographers perform the procedure in a few minutes.

An ultrasound exam tells of the state of your pregnancy in real-time, much like a snapshot of your womb. Its results are guaranteed to reflect the actuality and condition of your body, including the embryo’s size, position, and formation. In conjunction with the time of the Last Monthly Period or LMP, ultrasounds make known if the fetus is between five and six weeks for exact gestation age.

That’s because gestation age is counted starting from the first day of your LMP, not when you missed the period. Having an early ultrasound also makes it easier to calculate when you conceived. Later tests are less accurate, especially after the first two months of pregnancy.

Your LMP tells of when ovulation was taking place, seeing as that’s the only chance you had for conception. Estimates compare to what’s considered anormal pregnancyor how a fetus is supposed to grow. However, your baby can be larger than it’s expected, often due to genetics or macrosomia. That’s a condition that enlarges the fetus, possibly due to gestational diabetes.

What Should I Expect During an Ultrasound Exam to Determine the Conception Date?

Ultrasound is used to determine when you conceived, especially if you had irregular periods before getting pregnant. During the first and early second trimesters, all healthy fetuses grow simultaneously, making ultrasound more accurate than other methods. Your baby’s gestational age is the one that doctors go with to determine your date of conception, seeing as ovulation isn’t an exact science.

The sonogram technician can perform a trans-abdominal or trans-vaginal test during the ultrasound exam. In the former, a gel is spread over your lower abdomen, and a transducer is introduced to examine the fetus using sonar waves. An accompanying computer screen projects the image of your fetus, from which measurements are taken to gauge gestational and fetal age.

Transvaginal ultrasounds give a more accurate conception date, especially in the early days when you’re unsure of your LMP dates. Another of the initial ways your doctor can tell how far along you are is through detecting the fetus’s heartbeat. That’s because the heart starts to beat at between five to six weeks, a signal that indicates your pregnancy’s age.

What’s the Crown to Rump Ratio, and How Is It Measured?

Overall, an ultrasound will provide a more accurate estimate of when you conceived than relying on counting LMP dates or tracking basal temperature. The sonogram technician takes measurements of your baby’s arms, legs, torso, and head to determine gestational age. Based on these ‘crown to rump’ lengths, you can also arrive at your pregnancy’s fetal age, which then points to the time of conception.

During ovulation, which can either be long or short, the sperm fertilizes your ovum, which results in conception. The fused cell will travel down your fallopian tubes to embed itself on your uterus wall, forming a zygote. An ultrasound can detect and take measurements once this forms a mucus layer and grows into an embryo.

In the 8th week of pregnancy, the embryo enters the fetal stage, and that’s when most doctors refer you for an ultrasound exam. The following crown to rump lengths represent the gestational age of your baby, including;

  • 1.22 inches is ten weeks of pregnancy
  • 11 weeks gestational age is 1.61 inches  
  • 2.13-inch crown to rump length is 12 weeks pregnant
  • Between 2.19 and 2.64 inches is 13 weeks of pregnancy
  • 3.42 inches means you’re 14 weeks pregnant
  • A 15-week pregnancy has a crown to rump length of 3.98 inches

When Is the Best Time to Have an Ultrasound Scan That Determines When I Conceived?

Typically, it’s not until you are five to six weeks pregnant that the fetus is visible. That’s the best time to schedule an ultrasound to determine the viability of your pregnancy and estimate conception and due dates. By the time you’re between 18 and 28 weeks along, the margin of error will have increased to plus or minus two weeks; while longer than that, you’ll be counting three weeks off.

You can also use a conception calculator, together with your ultrasound results, for a more accurate date of conception. That’s because tracking your LMP or any other method is an assumption, while the ultrasound showcases your baby’s actual growth. It’s when your pregnancy crosses the 18th week that accuracy falls since the crown to rump ratio only works well with small fetuses.

Ovulation happens between the 11th and 14th day of your menstrual period, and conception can occur within the first 24 hours. Your date of ovulating can also be used as a possible time of conceiving, considering that you’ve accounted for the luteal phase. That’s the stage when your egg leaves the ovary, and progesterone is secreted from the follicles.

If your cycle is longer, this stage remains constant, pushing your ovulation date ahead of the 14th after the first day of your LMP. That’s why having an irregular period or forgetting when your period started makes calculations dicey. But with ultrasound crown to rump ratio results, you can narrow down the determination to arrive at an accurate conception time.


Gestation age is measured in weeks, starting from the first day of your last monthly period before you became pregnant. That’s the results given to answer the question of when did I conceive based on ultrasound. Together with the ‘crown to rump length’ of your fetus, a sonogram technician will make adjustments that pinpoint the exact time you conceived.