How we count pregnancy weeks?

Congratulations on your pregnancy! Or maybe you’re just curious about how counting pregnancy weeks work (or maybe your friend is pregnant and you don’t want to sound so out of place, huh?). Either way, understanding the ins and outs of counting pregnancy weeks can be a bit daunting (there’s more math involved than I expected) – but never fear, we’ve got you covered. In this article, we’ll go over all the basics (without making it feel like a boring old anatomy class) from what exactly constitutes as a week in pregnancy to why due dates aren’t set in stone.

The Basics: What Exactly Is a Week in Pregnancy?

To start off our journey, let’s define what we mean when it comes to counting weeks during pregnancy (more specifically by standard convention for obstetrics). When discussing how far along someone is in their gestational period (fancy word alert!), most doctors use the first day of someone’s last menstrual period (LMP) as the indicator for day one. Why LMP instead of conception date? Well, LMP tends to be easier to pinpoint since many people might not know exactly when they ovulated/conceived.

So if your doctor tells you that you’re “eight weeks pregnant,” that means eight full weeks have passed since the first day of your last menstrual period. Conversely, if someone tells you that they are at “32 weeks,” that typically refers to 32 whole completed gestational periods from when they had their last period.

Wait… Aren’t There Different Types Of Weeks To Count During Pregancy?

We’re anticipating some raised eyebrows here, but yes there actually are different types of ways people count ‘weeks’ when tracking pregnancies – which can understandably become super confusing:

Embryonic vs Gestational Age

When referring strictly towards an embryo/fetus’s progression in development, embryonic age is utilized. Embryonic ages are typically calculated from the date of conception or fertilization whereas gestational ages count weeks using dating to LMP as described earlier (which will essentially be 2 weeks ahead). So if someone says “my embryo is at 8 weeks,” that means eight full developments/changes have taken place since fertilization/conception occurred.


Rather than counting developmental milestones or specific dates, trimesters split a pregnancy into three distinct portions: first (0-12ish) creeps towards morning sickness land; second (a little reprieve), and third (cue difficulty sleeping all night and kicking just about every organ).

Why All This Counting Is Important In The First Place

Biology time! (Brace yourselves) During the initial stages of pregnancy, the zygote travels down the fallopian tubes until it eventually implants within the uterine wall (it’s like HOUSE HUNTERS but for embryos y’all: trying to find their perfect nesting location). From there on out, cell division keeps mum’s uterus housing this growing clump of cells which eventually develops into a tail-kicking fetus.

But developing isn’t just an altogether straightforward process – hence why babies may take different timelines in terms of full term carry-over periods. Knowing how far along one person is during pregnancy can assist with knowing when particular changes should occur such as organ formation/birth events etc., Also important? Keeping track helps tell whether potential issues require management immediately versus waiting/watchful vigilance.

Understanding Fetal Development Vs When Certain Tests Are Done

There are certain prenatal screenings/particular tests conducted depending upon fetal growth/maturation milestones. Some options include:


Ultrasounds use soundwaves allowing doctors a visualisation window into Momma’s belly helping peep development/growth markers overall competency/recovery progress.

Month-to-Month Developmental Guidance

Once certain gestational milestones are hit, physicians may install respective checklists pertinent for that timeframe (may include advice on specific nutrition requirements many moms crave during the period). Monthly guidance can help you plus your care-giving team tackle concurrent complications as they arise. Never be afraid to ask about any minor concerns because at this stage it is better safe than sorry.

What About Due Dates? Are They Really Set In Stone?

Due dates serve as a tool for orientation instead of concrete delivery deadlines (although wouldn’t babies making appointments be so much easier?). Typically due dates fall roughly 40 weeks from the first day of someone’s last period – but in actuality, only around 5% of babies arrive right on schedule.

Calculating Delivery Window

Doctors may calculate an expected window extension (or EDD) using formulas such as Naegle’s rule or ultrasonic measurements before baby pops out suddenly one fine evening drooling herself and screeching gibberish at everyone nearby!

Changing Your Due Date

If changes arise within Momma/marsupial co-habiting premises- physical/emotional factors restricting conventional thresholds/experiencing earliness or delay beyond likely projected timescales entirely reversible/non-treatable, alternative adjustments can become necessary which includes amending original aim areas too! (and just when we thought our schedules were difficult to keep with!)

Wrapping Up: How To Stay Sane When Trying To Count Pregnancy Weeks

Pregnancy really teaches us how unpredictable life could become despite our best intentions – biology will do what biology does! However keeping track/helping understand developments often aids potential discomfort/trauma management especially given child birth usually leaving newborns requiring constant care/attention well after birthing proper.

Despite all previously mentioned info/potentially confusing terminology used throughout this article: prenatal/gynecological care teams/etc will remember all the key performance metrics outlined in here. They’ll work with you right from calculating conception periods to creating individualized care plans for your unique pregnancy journey!

Whether you’re counting down weeks until your little one comes (cue lovely baby snuggles!) or simply curious about how it all works (and just why are trimester breakdowns made so hard to comprehend anyways?), we hope this guide helped cleared up any confusion – and provided a laugh or two along the way!

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