Estimating Baby Size Week by Week When Your Last Period Date Is Unknown

That Positive Test, But No Calendar to Count From

You're holding a positive pregnancy test. The excitement hits. Then the practical questions flood in: how far along am I? When did I conceive? What's my baby's size right now?

But here's the problem—you can't remember your last period. Maybe your cycles are irregular. Maybe you were on birth control. Maybe you just weren't tracking. You pull up a pregnancy calculator online, and every single one asks for the same thing: "First day of your last menstrual period (LMP)."

You stare at the field. You don't have an answer.

Why the Missing LMP Throws Off Everything

Most pregnancy calculators use a straightforward formula: take your LMP, add 280 days (40 weeks), and that's your estimated due date. From there, they calculate your current week and show you baby size week by week—whether your little one is the size of a poppy seed or a lime.

The entire system hinges on knowing that one date.

Here's why that's a problem for many women:

Irregular cycles: If your cycle varies between 28 and 45 days, ovulation timing shifts dramatically. An LMP-based calculation assumes a textbook 28-day cycle with ovulation on day 14.

Birth control: Hormonal contraceptives can mask or alter bleeding patterns. That "period" you remember might have been withdrawal bleeding, not a true menstrual cycle.

Postpartum or breastfeeding: If you haven't resumed regular cycles after a previous pregnancy, there may be no LMP to reference at all.

Simply not tracking: Not everyone marks their period on a calendar. Life is busy.

When you can't provide an LMP, standard pregnancy week calculators can't place you on the timeline. And without that placement, you can't track fetal development, estimate your due date, or follow along with baby size comparisons week by week.

The Solution: Reverse-Engineering Your Pregnancy Timeline

Here's the good news. You don't actually need your LMP to figure out how far along you are. You need a different anchor point—and then you work forward from there.

This is called ultrasound dating, and it's actually more accurate than LMP-based calculation in many cases, especially early in pregnancy.

Let me walk you through the complete workflow, step by step.

Step 1: Get an Early Ultrasound (Ideally Between 7–10 Weeks)

The single most important thing you can do when you don't know your LMP is to schedule an early ultrasound. The earlier, the better—here's why.

In the first trimester, babies grow at a remarkably predictable rate. This predictability means your ultrasound technician can measure the baby and determine gestational age with surprising precision. After about 12–13 weeks, individual growth differences start to emerge, and the accuracy drops.

The key measurement taken during this ultrasound is called the crown-rump length (CRL)—the distance from the top of the baby's head to the bottom of the buttocks. It's measured in millimeters and corresponds directly to gestational age.

Step 2: Understand What the CRL Number Tells You

Here's a concrete example to make this real.

Let's say your ultrasound technician measures your baby's CRL at 23 millimeters. According to standard fetal biometry charts, a CRL of 23mm corresponds to approximately 8 weeks and 6 days gestational age.

That number—8 weeks 6 days—is your new anchor point. It replaces the missing LMP entirely.

From this measurement, your healthcare provider can:

• Determine your estimated due date (EDD) by counting forward
• Place you accurately on the pregnancy week by week timeline
• Track baby size and development milestones going forward

The accuracy here is impressive. A first-trimester CRL measurement is typically accurate to within 5–7 days. Compare that to LMP-based dating, which can be off by two weeks or more for women with irregular cycles.

Step 3: Calculate Your Due Date From the Ultrasound

Once you have your gestational age from the ultrasound, calculating your due date is straightforward math.

Here's how it works using our example:

• Ultrasound says you're 8 weeks 6 days on, say, March 15th
• A full-term pregnancy is 40 weeks
• You have 31 weeks and 1 day remaining (40 weeks minus 8 weeks 6 days)
• Count forward 31 weeks and 1 day from March 15th
• Your estimated due date lands around October 24th

Most pregnancy calculators will do this math for you if you can input your gestational age instead of an LMP. Look for calculators that offer an "ultrasound dating" or "known gestational age" input option rather than requiring a last period date.

Step 4: Map Baby Size Week by Week From Your Anchor Date

Now that you have a reliable gestational age and due date, you can track your baby's growth week by week with confidence.

Using our example (8 weeks 6 days on March 15th):

• At 9 weeks (around March 16th): Your baby is roughly the size of a cherry, about 0.9 inches and weighing less than a tenth of an ounce
• At 12 weeks (around April 6th): About the size of a lime, 2.1 inches, around half an ounce
• At 20 weeks (around June 1st): The size of a banana, approximately 6.5 inches and 10 ounces
• At 30 weeks (around August 10th): About the size of a large cabbage, 15.7 inches and nearly 3 pounds
• At 40 weeks (around October 24th): Full-term, around 18–20 inches and 6–9 pounds

The beauty of this approach is that each weekly baby size milestone now has a specific date attached to it. You're not guessing. You're tracking against a clinically validated starting point.

Step 5: Cross-Check With Other Indicators

While the ultrasound is your gold standard, it doesn't hurt to verify your timeline against other clues your body and medical records provide. Think of these as supporting evidence rather than primary dating tools—they can confirm you're in the right ballpark or flag a discrepancy worth discussing with your provider.

hCG Blood Levels

Human chorionic gonadotropin (hCG) hormone levels follow a general trajectory in early pregnancy. A quantitative hCG blood draw can give your provider a range that corresponds roughly to gestational age. While hCG numbers vary widely between individuals, a single reading paired with a follow-up test 48 hours later can show whether levels are rising appropriately. This isn't precise enough to replace ultrasound dating, but it can corroborate the timeline.

Physical Symptoms and Milestones

Certain pregnancy milestones tend to occur within expected windows:

• First positive pregnancy test: Typically around 3–4 weeks gestational age, though sensitive early-detection tests can pick up hCG as early as 10 days past ovulation.
• Morning sickness onset: Usually kicks in around 6 weeks. If you remember when nausea first started, that can serve as a rough checkpoint.
• First fetal heartbeat on ultrasound: Generally visible around 6–7 weeks via transvaginal ultrasound.
• Quickening (first fetal movement): Usually felt between 16–22 weeks, earlier for second-time mothers.
• Fundal height measurement: After 20 weeks, the distance from your pubic bone to the top of your uterus in centimeters roughly correlates with gestational weeks.

If any of these milestones line up with your ultrasound-based timeline, you can feel confident in your dating. If something seems off by more than a week or two, bring it up at your next appointment.

What If Your Ultrasound Dating Conflicts With Later Scans?

Here's a scenario that confuses many expectant mothers. You had an early ultrasound at 8 weeks that gave you a firm due date. Then at your 20-week anatomy scan, the technician mentions the baby is measuring "a few days ahead" or "a little behind." Suddenly you're second-guessing everything.

Here's what you need to understand: your earliest ultrasound is the most accurate dating tool. After the first trimester, babies begin growing at individual rates. A 20-week scan might show your baby is in the 60th percentile for size, but that doesn't mean your due date was wrong—it means your baby is simply growing slightly faster than average.

Medical guidelines are clear on this. The American College of Obstetricians and Gynecologists (ACOG) recommends that if a first-trimester ultrasound dating differs from LMP-based dating by more than 5–7 days, the ultrasound date should be used. And once an early ultrasound has established your due date, later scans should not change it. They're used to monitor growth, not recalibrate your timeline.

So if your 8-week ultrasound said October 24th is your due date, stick with that. Don't let a later scan shake your confidence or push you into a different week of your pregnancy tracker.

Practical Tips for Tracking Without an LMP

Now that you have a reliable anchor date, here's how to stay on track throughout your pregnancy:

Use the Right Type of Calculator

Many online pregnancy calculators are rigid—they only accept an LMP input. Look for tools that let you enter your due date directly or input a known gestational age. Once you plug in your ultrasound-derived due date, these calculators will generate the full week-by-week breakdown, including baby size comparisons, development milestones, and trimester transitions.

Set Calendar Reminders for Weekly Milestones

Since you're tracking from a clinical anchor point rather than a remembered period date, set a recurring weekly reminder on your phone. Mark it for the day of the week your gestational age advances. For example, if your March 15th ultrasound placed you at 8 weeks 6 days, every Saturday marks a new week. Use this trigger to check your pregnancy app or calculator for the latest baby size update.

Keep a Pregnancy Journal

Without an LMP to reference, you might find it harder to answer when friends, family, or even medical staff ask how far along you are. Keep a small note on your phone with your current gestational age and due date. Update it weekly. This sounds simple, but it eliminates the awkward moment of trying to do mental math in the waiting room.

Bring Your Ultrasound Report to Every Appointment

Different providers and different offices sometimes use different dating methods. If you switch providers mid-pregnancy or visit a new clinic, they may default to LMP-based dating if you can't provide one. Having your early ultrasound report on hand ensures your established due date carries forward correctly.

When to Contact Your Provider About Dating Concerns

While the ultrasound-based approach is reliable, there are a few situations where you should reach out to your healthcare provider:

• You never had a first-trimester ultrasound. If your first scan is after 13 weeks, dating accuracy decreases. Your provider may adjust your due date based on a combination of measurements and other factors.
• Your measurements are consistently off by two weeks or more. If fundal height or later ultrasound measurements consistently fall outside the expected range, your provider may investigate growth restrictions or other concerns.
• You have a history of complications. Prior preterm birth, gestational diabetes, or growth issues may warrant closer monitoring regardless of how your pregnancy was dated.

The Bottom Line

Not knowing your last period date feels like a roadblock, but it doesn't have to derail your ability to track your pregnancy. The reality is that ultrasound dating—especially in the first trimester—is often more accurate than LMP-based calculations, even for women with perfect cycle records. Babies grow at predictable rates early on, and that CRL measurement gives you a clinical anchor point that no forgotten calendar date can match.

Here's your action plan in summary:

• Schedule an early ultrasound, ideally between 7–10 weeks
• Get your CRL measurement and corresponding gestational age
• Calculate your due date by counting forward to 40 weeks
• Use that due date in any pregnancy calculator to track baby size week by week
• Cross-check with hCG levels and symptom milestones for added confidence
• Stick with your original ultrasound due date even if later scans show slight size variations

Your pregnancy journey doesn't require perfect memory of your menstrual history. It requires a reliable clinical measurement and the right tools to interpret it. With an early ultrasound and a due-date-based calculator, you can follow your baby's growth from poppy seed to pumpkin with the same confidence as anyone else—maybe more.