Pregnancy Calculator

Due Date Calculator

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Dr. Penman’s Due Date Calculator

When booking a baby scan, it’s important to have an idea of how far along in pregnancy you are. This is because this date helps plan gestation-appropriate prenatal care, testing, and baby scans.

You can have your first baby scan with Dr. Penman as early as 6 weeks of pregnancy. This scan is called the Reassurance Scan and hopes to give you peace of mind that all is well with baby.

Dr. Penman’s Due Date Calculator should help give you an idea of what week of pregnancy you are in. It also offers possible dates for each of the pregnancy scans and tests Dr. Penman provides.

If you have any questions, please contact Helen, Dr. Penman’s PA.



Enter the date of the first day of your last menstrual period (LMP):



Probable date of conception:


Fetal Age Today:


Viability Scan Date:


(7 weeks)

Best date range for NT scan:


(12 weeks 3 days to 13 weeks 3 days)

Anomaly Scan Date:


(19 weeks)

Best time for routine anomaly ultrasound:


(18 to 20 weeks)

Estimated Due Date:



How is my estimated due date calculated?

Your due date is calculated as 40 weeks (280 days) from the first day of your last period.

In reality, this date is referred to as the ‘estimated due date’ as it is just that – only an estimate. It assumes you have a 28-day menstrual cycle. It doesn’t take irregular or longer menstrual cycles into account. A regular menstrual cycle that is considered ‘typical’ can range from 28 to 40 days.

Baby scans are the most reliable way to calculate your estimated due date. The earliest baby scan Dr. Penman offers is at 6 weeks and is called the Reassurance Scan (6 – 10 weeks).

Whilst it doesn’t happen often, your due date can change by a few days or a few weeks. Whether it changes and by how much depends on a few factors, such as when you had your first baby scan and, among other things, whether or not you had regular periods before getting pregnant.

If due dates are only estimates, why are they so important?

Due dates are more important as a guideline for the care of mother and baby. They also give you a guide as to when you can have certain prenatal tests, such as the Harmony Test.

The Harmony Test is a blood test. It tests for Down Syndrome (trisomy 21) and other common fetal chromosome abnormalities such as Edwards Syndrome (trisomy 18) and Patau Syndrome (trisomy 13) along with 300 other genetic abnormalities such as cystic fibrosis. You can have the Harmony Test as early as 10 weeks of pregnancy.

This test can be done in naturally conceived pregnancies as well as IVF pregnancies and in both singleton and twin pregnancies. It will also tell if your baby is a boy or girl.

Why isn’t my conception date used in estimating my due date?

Conception is hard to pin down to one day, so that’s why it’s not used in estimating your due date.

This is because sperm can survive inside you for a few days, so fertilization can occur anytime up to 5 days after you had sex. The first day of your last period is a fixed day so that’s why we use that to calculate your estimated due date.

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