How to Tell When You Conceived

  Photo by  Samantha Gades  on  Unsplash
Photo by Samantha Gades  on Unsplash

When did I get pregnant?

It can be difficult to pinpoint exactly when you became pregnant. It’s much easier if you have regular, predictable menstrual cycles or if you were tracking your ovulation.

If you know the start date of your last period, you can probably figure out an estimated window of when you conceived, based on when you were ovulating.

For example, if you started your last menstrual period (LMP) on February 1st and you have a regular, 28-day cycle, then you would have ovulated on or around February 15th. If you had sex between February 10 and 15th, you conceived during that time. Implantation most likely occurred around February 24th and your baby will be due on November 7th.

If your periods are irregular, you were on hormonal birth control, or you weren’t tracking ovulation, then it can be very difficult to know the exact time of conception.

Why do I need to know when I conceived?

In order to determine an accurate estimated due date (EDD), your doctor will need to know information like the start date of your last menstrual period (LMP) or when you ovulated. Having an accurate EDD will affect most of the decisions you face during your pregnancy.

With an accurate EDD, your doctor is able to plan the timing for certain antenatal tests. Additionally, she will be able to monitor the appropriateness of your baby’s growth. It will also help your doctor intervene if a pre-term or post-term birth seems likely.

Essentially, an accurate EDD will prevent you from giving birth too early or too late. It will also allow your doctor to test you for the right conditions at the right times.

What if I don’t know when I conceived?

If you have no idea when your LMP or ovulation happened, rest assured. There are many other methods to determine an accurate EDD, especially as your pregnancy progresses.

  • First Trimester Ultrasound--This is, in fact, the most accurate way to determine the age of your fetus, and therefore your due date. High-quality ultrasound measurements in the first 14 weeks of pregnancy are the best way to determine your due date.
  • Fundal Height Measurements–Between 12 and 14 weeks of pregnancy, your doctor will begin measuring your stomach to see if your “fundal height” is in line with your EDD. Your doctor will measure you from the top of your pelvic bone (symphysis pubis) to the top of your uterus (fundus). These measurements allow your doctor to see if Baby is growing as expected. Between weeks 20 and 36, your fundal height measurement should be withing 2cm of the weeks of gestation. For example, at 30 weeks pregnant, your fundal height should be between 28 and 32cm. If you’re measuring significantly ahead or behind, you doctor may want to do another ultrasound to determine if adjusting your EDD if necessary. Fundal height measurements have a vast range of accuracy, ranging from 17-93%, so additional information from an ultrasound is often needed. They are fairly subjective as some providers do not take the mother’s height, weight, or body shape into account. Additionally, Fundal height measurements need to be taken by the same provider every time for the highest level of accuracy.
  • Baby’s Estimated Measurements-Around 20 weeks of gestation, you will most likely be sent to a specialist for an Anatomy Scan, which is a high-quality ultrasound. During the anatomy scan, you will be able to find out your baby’s sex. Additionally, your doctors will measure nearly everything on your baby from his brain to his spine to his face. When they record these measurements altogether, your providers will have a very good idea of your baby’s gestational age and your due date. This is also when you’ll be notified of any potential fetal abnormalities. Don’t panic is something seems off during your anatomy scan. If something is amiss, you’ll be back for another scan at 28 weeks. By then, whatever issue you were facing has probably fixed itself. In the rare case, when you and your baby are facing a serious medical issue, you will be referred to a genetic counselor. Again, this is very rare.
  • ART (Assisted Reproductive Technology) Pregnancy Measures–If you became pregnant through In Vitro Fertilization (IVF) or another ART technique, your doctor will need to take those circumstances into consideration. The date of transfer and the age of the embryo will need to be known to determine an accurate EDD.

What if my doctors change my due date?

Because of the advances in reproductive and ultrasound technologies, it is very rare for a doctor to change your estimated due date. If dramatic differences are noticed at some point during your pregnancy, your doctor will discuss changing your due date with you. Any changes need to be clearly recorded in your medical file. This way, if you need to go to Labor and Delivery without your primary care provider, they will have your most accurate and up to date information. This prevents hospitals from inducing labor too early. It also helps them stall a preterm labor.

I know my due date, but I still don’t know when I got pregnant.

If you’re in this situation, you can do the same ovulation math, but in reverse. For example, if you know you are due on November 7th, you could count back 38 weeks and discover that you most likely conceived sometime between February 10th and 15th. There are also many online calculators you can use if you don’t want to do the math in your head. Once you know your due date, you can count back 38 weeks to find when you were most likely to conceive. Another way to think about it is based on your last period. If you know when you last started your period, look forward about two weeks. Around that two week mark is when you were most likely to conceive.

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