When did I conceive? is one of the most frequently asked questions at ultrasound appointments at The Pregnancy Network.
Your provider will generally use the LMP method—the first date of your last menstrual period—to determine how far along you are in your pregnancy. While this is not the date of conception, it is still considered a reliable method for dating your pregnancy.
LMP: First Day of Last Menstrual Period
Though a woman cannot conceive on the first day of her period, this is considered the most reliable starting point to estimate how many weeks pregnant you are. This is because it is a known, trackable factor.
Registered nurse Lauren M. explains,
Basically, we date pregnancy off of the first day of your last menstrual period. We do that because it is the last known date that we can confirm something happened. Obviously, you are not pregnant while you are on your period. You were probably not pregnant the week after your period. But, that is how we date pregnancy because it is a known factor. What we do not know for sure is when you ovulated, and when you conceived.
Unlike the last period when we know exactly when it started, ovulation and conception are more like windows where we have this idea of when it happened, but we can’t pinpoint it exactly.
Click below to hear the full explanation.
Conception
The moment an egg is fertilized is conception. This moment is the exact starting point of your pregnancy but can occur up to 5 days after having sex. (Sperm remains viable up to 5 days after ejaculation.) If a woman ovulates during this 5-day period, then conception can occur. An egg can be fertilized in the 12 to 24-hour window after it is released into the ovary. Lauren summarizes,
“You actually could have sex before you ovulated, and still get pregnant after your ovulation…You can also have sex a few days after ovulation and still get pregnant as well.”
This explains why you may be considered 4 weeks pregnant by the LMP method, but only have conceived two weeks ago.
Click below to watch a short video of Lauren explaining the difference between conception and LMP dating.
Early Ultrasound
At an early pregnancy ultrasound, your nurse will measure the developing fetus and determine the crown-rump length by measuring the baby from head to bottom. Lauren explains that this measurement, “tells us how far along you are compared to what that last menstrual period date is.’
Lauren shared that this date can give you a more accurate idea of how far along you are and provide an estimated due date. This due date is still considered a window that includes a few days before and after that estimated date.
If you’ve had multiple partners within the window of ovulation and conception, you may be hoping that an early pregnancy ultrasound would help determine which partner is the father of your baby.. Unfortunately, even measuring the size of your baby cannot pinpoint the exact date of conception.
What To Do Next?
Think you might be pregnant? At The Pregnancy Network, we can offer you a free pregnancy test and a same-day* limited ultrasound. An ultrasound will help determine how far along you are in your pregnancy, and even more importantly, confirm that the pregnancy is developing in the uterus and not ectopic.
All medical services are provided by our staff licensed registered nurses. Make an appointment today for compassionate, confidential care. We’re here for you to help you make an informed decision about your pregnancy.
*Same-day limited obstetric ultrasounds per availability.
Kacey Minor
Kacey Minor is the Communications Manager at The Pregnancy Network.