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Questions about conception dating are often the first we are asked, usually right after getting a positive pregnancy test: “When did I get pregnant?” they ask. It’s just one of those things women want to know – either in general or because they have additional questions, usually about paternity.
Conception occurs when the egg is fertilized by a sperm. This is the starting point of pregnancy. There are two ways to determine your conception window.
The LMP method
Your LMP is the first day of your last menstrual period, and you will be asked this date throughout your pregnancy.
Knowing your LMP will give you a range of when you likely conceived. The typical menstrual cycle – meaning from the first day of one period to the first day of the next period – is 28 days.
Ovulation usually occurs 14 days before your next period begins. Ovulation is when an egg is released from your ovary; once the egg is released there is a pregnancy window of time in which the egg can be fertilized.
An egg survives for 12-24 hours after ovulation; if not fertilized by a sperm during this time period, it is reabsorbed by your body.
The LMP method seems pretty straight forward: if you know your LMP you can predict your next LMP and count 14 days back to figure out an idea of your conception date. However, there are some variables at play.
If you track your periods – either on a calendar or on an app – you may know your LMP quickly and confidently. It may be a more difficult question to answer accurately if your periods are irregular. It may be that you just don’t remember when you had your last menstrual period (that’s ok, read on).
Some women have cycles longer than 28 days and some have cycles shorter than 28 days. Some women’s cycles vary in length from one month to the next and some only have a cycle a few times a year. These variances will affect your ovulation window.
If you are a teenager you may not have regular cycles, which will make this LMP method less reliable.
If you’re perimenopausal, irregular cycles may also make this LMP method unreliable.
The sperm also bring their own variables. Unlike the egg, sperm can survive for several days inside your body. So, if you’re trying to establish conception dating for paternity purposes, understand that if you have sex a few days before you ovulate, the sperm could still be in your body and fertilize your egg when ovulation occurs.
That’s why you will often hear that you have a “fertility window” – a time period of days where you could get pregnant each month – rather than a fertility day.
The Ultrasound Method
Early pregnancy ultrasounds are another way to help with conception dating. In early pregnancy, fetal development is fairly standard, so measuring the size of the fetus during this stage gives us a pretty accurate dating for the pregnancy. Once you have the age of the pregnancy, you can work backward to determine the conception window.
Transvaginal ultrasounds are usually performed up to 12 weeks to ensure you have the most accurate information. If you’ve never had an ultrasound, the thought of a transvaginal ultrasound may make you uncomfortable, or even fearful.
Transvaginal ultrasounds are not painful, but you main feel some pressure or discomfort. For women experiencing their first pregnancy, we find that usually the anticipation is worse than the procedure.
Ultrasound is the most requested service we have here at Alcove Health Women’s Clinic. Women of all ages, whether or not it’s their first pregnancy, want this ultrasound because they know it will give them information they cannot get otherwise.
Our nurses are compassionate and explain every step prior to beginning the ultrasound. As with all our services, you determine which services you receive based on your consent.
One reason some women ask about conception dating is to figure out their due date. If you know your LMP and you have regular cycles, you can use our Due Date Calculator to figure it out right here:
If you’re facing an unplanned pregnancy, just thinking about all this may make you panic. But don’t! Even though it may feel overwhelming to have more questions than answers, know that you’re not alone. We’re here to help you figure this out and get your questions answered.
Don’t stress over not knowing your LMP, or not understanding conception dating information, or even not wanting to discuss your pregnancy with anyone else until you get some answers. Whatever your questions, whatever your experiences, we want to help.
Here at Alcove Health Women’s Clinic, we provide free and confidential services so you can get the answers you need to plan your next step and choose your next conversations. None of our services require parental permission, either. You deserve safe and accurate medical & advocacy services to protect your reproductive health and learn more about conception dating. Call or visit our appointment page today to request your appointment with Alcove Health.
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