Calculating Pregnancy Age and Due Date From Your LMP

The midwife says you are eight weeks. You know exactly when this happened, and it was not eight weeks ago.

Both of you are right. Pregnancy is counted from the first day of your last period, which puts the number roughly a fortnight ahead of the biology.

The count starts before you conceived

Your LMP is not day one of the pregnancy.

You conceived around two weeks after that period started, at ovulation. But almost everyone knows the date their last period began, and almost nobody knows the day they ovulated, so the LMP is what gets used.

So at four weeks pregnant, there has been an embryo for about two. It is a practical convention, not a claim about biology.

Working out how far along you are

Count the weeks from the first day of your last period to today.

Last period started on 1 January and today is 1 March? That is about nine weeks, so you are about nine weeks pregnant.

Weeks rather than months, because the milestones line up with weeks and the trimesters refuse to divide neatly into calendar months.

Working out your due date

The standard formula, called Naegele's rule:

First day of your last period, minus three months, plus seven days, plus one year.

Say it began on 10 May 2026:

• Minus three months: 10 February 2026
• Plus seven days: 17 February 2026
• Plus one year: 17 February 2027

That is your estimated date. Clinics use a rotating pregnancy wheel, or software, which does exactly this and shows you the current gestational age at the same time.

Why your date may get moved

The formula assumes a 28-day cycle with ovulation on day 14. Plenty of us have neither.

If your cycle runs consistently longer or shorter, the LMP date is off by roughly that difference.

An early scan beats the arithmetic. A dating scan in the first trimester measures the embryo directly, and where it disagrees with the LMP by more than a few days, the scan usually wins.

So if a midwife revises your date, nothing has gone wrong. That is the system working.

About one baby in twenty turns up on the date

The due date is the middle of a range, not an appointment.

Most babies arrive between 37 and 42 weeks. Early or late is the norm, not the exception, which is worth saying out loud to whoever plans to text you daily from week 39.

If you have no idea when your last period was

Very common, especially with irregular cycles, coming off contraception, or a pregnancy that follows soon after the last one.

An early scan solves it entirely. Book the first appointment and let the scan do the dating.

One thing worth timing

Knowing your dates makes it easier to start the things that want a head start.

Stretch marks form fastest in the second and third trimesters, when the bump grows quickest. Getting into a moisturising habit before that stretch begins is easier than trying to fade marks afterwards.

Mama's Choice Stretch Mark Cream is water-based, light, and made for pregnancy and breastfeeding.

Honestly though: no moisturiser has been shown to prevent stretch marks. What it does is keep your skin comfortable while it stretches, which is worth having on its own.

When to see a midwife or doctor

• As soon as you get a positive test, to book antenatal care
• Bleeding or cramping at any stage
• Sickness bad enough that you cannot keep fluids down
• Any uncertainty about dates, which the first scan settles

Booking early matters far more than calculating precisely. The scan will do better arithmetic than any formula.