How To Check Pregnancy Weeks At Home | Know Your Week Number

When you’re trying to pin down “what week am I,” the calendar can feel slippery. You might be tracking symptoms, booking your first visit, or trying to match what an app says with what your body feels. You can get a solid week estimate at home with one main date and a simple count.

This guide shows the most reliable at-home ways to work out your pregnancy weeks, what can shift the number, and how to keep your notes tidy for your first appointment.

How pregnancy weeks are counted

Pregnancy weeks are usually counted from the first day of your last menstrual period (LMP). This is called gestational age. It starts before conception because it uses a date most people can recall.

Clinics use this system because it lines up with the standard 40-week timeline. It can feel odd that the clock starts before conception, but it keeps all care teams speaking the same language across labs, scans, and appointments.

Checking pregnancy weeks at home with your last period

If you remember the first day of your last period, you can count your weeks in a few minutes.

Step-by-step counting

1. Write down the first day you started bleeding for your last period.
2. Count the days from that date to today.
3. Divide by 7 to get weeks, then keep the remainder as extra days.

Example: your last period started January 1 and today is February 26. That’s 56 days. 56 ÷ 7 = 8 weeks, 0 days. You’d write it as 8w0d.

What a home pregnancy test can and can’t tell you

Home pregnancy tests can confirm pregnancy by detecting hCG in urine. They can’t date your pregnancy week. A positive test tells you “pregnant,” not “how far along.”

Some tests claim to estimate weeks based on hormone levels. Hormone levels vary a lot between people, so those ranges can be off. Use them as a curiosity, not your dating method.

When your cycle is not 28 days

LMP counting assumes a regular cycle. Many people don’t have that. If your cycles are longer, shorter, or unpredictable, LMP dating can drift from your ovulation timing.

A quick way to sanity-check

• If your cycle is longer than 28 days, ovulation tends to happen later, so LMP dating can make you look farther along than you are.
• If your cycle is shorter than 28 days, ovulation tends to happen earlier, so LMP dating can make you look less far along than you are.

If your cycle length is steady, compare it with 28 days. A 32-day cycle is 4 days longer, so your ovulation timing may run about 4 days later than the “day 14” assumption. A 24-day cycle is 4 days shorter, so ovulation timing may run about 4 days earlier. This helps you understand why a clinic date might differ from your own count.

Dating pregnancy at home when you know conception timing

Some people have a clear conception window. This can happen when you track ovulation closely, had a single likely date of sex, or used assisted reproduction.

Ovulation tracking

If you logged an LH surge, basal body temperature shift, or other ovulation markers, you may have a strong idea of your ovulation day. Gestational age is commonly counted as two weeks more than conception age. So if conception happened 6 weeks ago, gestational age would be 8 weeks.

IVF and embryo transfer dates

With IVF, clinics date pregnancy using the embryo transfer date and embryo age at transfer. If IVF applies to you, keep your clinic’s date as your main reference, even if a generic calculator gives a different number.

Using a due date calculator when you have LMP

If you don’t want to do the count yourself, a due date calculator can do it instantly. The NHS calculator is based on the first day of your last period and notes that pregnancy length commonly falls between 37 and 42 weeks from that date. NHS due date calculator is a solid option.

When you use a calculator, save the date you entered (your LMP) and the due date it returns. If your clinician later updates your dates after an early scan, update your notes so your week count matches your medical record.

Taking an early ultrasound date into account

If you already had an early scan and you have the report, the gestational age printed on it is often used to confirm dating, especially when LMP is uncertain or cycles are irregular. If a scan date differs from your home count, most care plans follow the date your clinician sets in your chart.

The American College of Obstetricians and Gynecologists describes the common due date convention as 280 days after the first day of the last period and outlines when other dating inputs can adjust that baseline. ACOG’s methods for estimating the due date lays out the method and the logic behind updates.

Table 1: At-home ways to estimate pregnancy weeks

Method	What you need	Where it can drift
LMP calendar count	First day of last period, today’s date	Irregular cycles, uncertain LMP date, recent hormonal contraception
Due date calculator (LMP)	First day of last period	Same limits as LMP counting, plus data entry mistakes
Cycle-length sanity-check	Typical cycle length over a few months	Cycles that vary month to month, stress or illness shifting ovulation
Ovulation kit log	LH surge date, notes on timing	Surge timing varies; ovulation can be later
Basal temperature chart	Daily temperature log	Sleep disruption, fever, inconsistent timing
Known conception window	Single likely date of sex	Sperm can live several days; fertilization day may differ
IVF transfer dating	Transfer date and embryo age	Usually steady; entry mistakes happen
Early ultrasound report	Gestational age listed on the scan	Later scans can be less consistent for dating than early scans

How to handle “I don’t know my last period date”

Not knowing your LMP is normal. Some people have spotting, irregular bleeding, or cycles that are hard to label. When the date is fuzzy, aim for a date range you can share at your first visit.

Build a date range you can explain

• Pick the first day you remember bleeding that felt like a true period.
• Write down any later bleeding that was lighter or shorter than usual.
• Note recent birth, breastfeeding, or contraception changes.
• If you have ovulation tracking, write your earliest and latest likely ovulation days.

Count weeks from your earliest possible LMP and from your latest possible LMP. That gives you a window you can share when you book care.

Getting care early

Once you have a week estimate, use it to schedule care. The NHS notes that the first midwife appointment should happen before 10 weeks where possible. NHS guidance on antenatal care lays out what early appointments include.

Guideline evidence summaries point in the same direction. An NIH-hosted guideline summary notes that people who first contact antenatal care after 9+0 weeks should be offered a booking appointment within two weeks where possible. NIH summary on timing of first antenatal appointment describes the logic behind early booking.

Table 2: A simple weekly tracking plan

Pregnancy weeks	What to track at home	What to schedule or confirm
4–5	Test date, LMP date, symptom notes	Call for a first appointment window
6–7	Nausea triggers, hydration, sleep notes	Ask what records to bring (LMP, meds, prior conditions)
8–10	Food tolerance, cramps, spotting notes	Confirm dating plan if cycles are irregular
11–14	Energy changes, activity comfort	Confirm screening and lab timing offered in your area
15–20	Belly growth notes, early movement notes once felt	Plan mid-pregnancy scan timing if offered
21–28	Swelling notes, sleep position comfort	Ask about glucose screening timing if recommended
29–36	Movement pattern notes if your clinician asks for it	Confirm visit cadence and birth location logistics
37+	Contraction timing notes, fluid leak awareness	Know when to call or go in for evaluation

Common mistakes that change the week count

These slip-ups come up a lot, even with apps.

Counting from the last day of bleeding

Gestational age starts on day 1 of bleeding, not the last day. Using the last day can shift your count by several days.

Marking spotting as a period

Light spotting can happen for many reasons. If you count from a spotting day that was not a true period, your week count can be off.

Letting an app guess your dates

Apps can fill in missing data using past patterns. If your cycle changed, the guess can be wrong. Your own notes beat an auto-filled date.

When home dating should be treated as a placeholder

Home dating is a strong starting point. Treat it as a placeholder and seek medical care soon if any of these are true:

• You have heavy bleeding, fainting, shoulder pain, or severe one-sided pain.
• You have a history of ectopic pregnancy.
• You can’t tell what counts as a period because bleeding is irregular.
• You used fertility treatment and want the clinic date used from the start.

If urgent symptoms are present, get urgent care right away. This article can’t diagnose problems.

A one-page worksheet for your dates

Copy this into your notes app so you have the numbers ready when you book visits.

• LMP (first day): __________
• Today’s date: __________
• Days since LMP: __________
• Weeks + days: __________
• Due date estimate (LMP + 280 days): __________
• Cycle length notes: __________
• Ovulation tracking notes (if any): __________

Can I use “How To Check Pregnancy Weeks At Home” as my only method?

You can use your home count to get a week estimate and to book care. Once a clinician confirms your dates (often using an early scan when needed), use that medical record date as your reference for the rest of pregnancy.