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How come a conception date is counted as the actual day you became pregnant if you had intercourse before conception happened?

This question originally appeared on Quora. Answer by Camila S. Espinoza, Midwife.

In order to answer this question, there’s something you need to understand first.

Conception never happens immediately after ejaculation. In fact, it can take place up to five days after intercourse. Why? Because sperm can survive up to five days inside the female reproductive system, though it won’t stay in optimal condition, survival is possible. Gravity will play its part, as well as the hostile conditions in the vaginal canal and uterus, including a slightly acidic pH and a hyperactive immune response, among others.

After day three, an important amount of sperm will be dead, and the ability of the remaining ones to move will be compromised, but conception during day four and five is still a possibility. The oocyte (a cell in the ovary) can only be fertilized up to 24 hours after ovulation but there’s no easy way to predict when ovulation will occur. Not only can sperm simply wait, it can actually go up in waves, keeping a reservoir that will maintain a constant abundance of sperm until it runs out.

But let’s say ovulation took place a few hours before ejaculation. The oocyte is being moved by the cells in the inside walls of the corresponding fallopian tube, and will be passing the ampulla (the optimal location for conception) by the time sperm finds it. Even under these ideal conditions, it will still take hours before sperm can move from the posterior vaginal fornix (deepest point of the vaginal canal, where semen creates a little pool), through the cervix, up the uterus, and across the oviduct in order to meet the oocyte. And even after that, the sperm that will eventually lead to a successful fertilization won’t be among the first wave of sperm that reaches this meeting point.

No, you weren’t conceived by the fastest swimmer, but by the most opportunistic one. The job of those selfless soldiers in the first wave is limited to weakening the zona pellucida, the outer layer that protects the oocyte from being fecundated. Only after an important portion of it has been dissolved by the enzymes carried in the head of the first wave of sperm, will a dent be created, significantly thinning the barrier that the one that outsmarted the rest will need to overcome by the time of its arrival.

If that doesn’t sound like team effort (or utter betrayal) to you, I don’t know what will.

Now, to the question itself: What is conception date, and how do we, as health professionals, calculate it?

First of all, we don’t. Conception date is not important to us, because we don’t really need it. All we need is the date of your last period and we will use that in order to calculate weeks of pregnancy.

So, this is how it goes: You’ll give us a date, and/or we will calculate one based on measurements that are taken during the first ultrasound. These measurements will give us a certain amount of weeks and from that, we will simply count backwards using a gestational disk (that round thing we spin left and right during your prenatal appointments) or a calendar to determine an operational date of last period. We’ll use one or the other depending on how confident you are of the date, how early the ultrasound was performed, and the amount of deviation in days between one and the other.

But I wasn’t pregnant during my last period , you’ll say. And yes, that’s true. Pregnancy didn’t even start right after conception, but approximately seven to nine days later, when implantation was successfully completed.

So What Is Conception Date?

Conception date is basically an estimation of when ovulation might have taken place during the menstrual cycle that lead to pregnancy. Most gestational discs (and pregnancy apps) take a standard 28 day cycle and estimate the date of ovulation as an event that happened at day 14 (counting from the date of your last period). Is this accurate? Absolutely not - it couldn’t be further from it because the date of ovulation varies according to a vast list of factors. The most important factor to consider is the total length of not just your menstrual cycles, but that one cycle in particular.

It’s commonly accepted that ovulation takes place two weeks before the next cycle starts, but pregnancy kind of wrecks that estimation by stopping menstruation completely, and we cannot accurately determine when conception occurred. So, we stick to the date of your last period to determine the weeks of pregnancy during the entire process.

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