How Pregnancy Due Dates Are Calculated (LMP, Conception, IVF and Ultrasound)

Pregnancy due dates are always estimates, not exact promises. Different methods – like using your last menstrual period (LMP), a known conception or IVF date, or an ultrasound – can give slightly different results, and that can be confusing. This guide explains the common ways due dates are calculated and how they connect to what you see in calculators like SoftBump.

Important: This is general information only. It cannot replace advice from your midwife, obstetrician or fertility team. Always discuss your own dates and care plan with a healthcare professional.

Gestational age vs fetal age

Before talking about methods, it helps to understand two terms doctors use:

• Gestational age is how far along the pregnancy is counted from the first day of your last menstrual period (LMP). This is the number used in appointments, ultrasounds and most due‑date discussions.

• Fetal age (sometimes called conception age) is the time since fertilisation. Because ovulation and conception usually happen about two weeks after the start of the period, fetal age is typically about two weeks less than gestational age.

So if your gestational age is 10 weeks, the baby's fetal age is around 8 weeks. Clinicians mostly use gestational age because it's tied to calendar dates in a way that works for scheduling tests and monitoring growth.

Method 1: Last menstrual period (LMP)

The most traditional way to estimate a due date is by LMP:

• You take the first day of your last menstrual period.

• You add 280 days (40 weeks) to that date to get an estimated due date.

This assumes a regular 28‑day cycle with ovulation around day 14. In reality, cycles can be shorter or longer, and ovulation isn't always exactly on day 14, so doctors sometimes adjust the estimate or rely more on ultrasound if the cycle length is very different.

Method 2: Known conception or ovulation date

Some people know roughly when conception happened – for example, through tracking ovulation closely, using certain fertility treatments, or having a very narrow window of possible dates:

• In that case, the due date can be estimated by adding about 266 days (38 weeks) from the conception date.

• This reflects the fact that gestational age counts two weeks before conception, while fetal age starts at conception itself.

Because exact conception is still hard to pin down in natural cycles, doctors often treat this as helpful context but still confirm timing with early ultrasound.

Method 3: IVF and embryo transfer dates

In IVF and other assisted reproduction, timing is much more precisely documented. Clinics know the age of the embryo and the exact transfer date:

• From the embryo transfer date and embryo age (for example, a day‑3 or day‑5 embryo), clinicians can calculate an estimated conception date.

• The due date is then estimated as conception + 266 days, and gestational age is aligned with that timeline.

This makes IVF‑based due dates one of the more precise methods, but even here the date is still an estimate – babies can arrive earlier or later than the calculated day.

Method 4: Ultrasound dating

Ultrasound can also be used to estimate how far along the pregnancy is, especially in the first trimester:

• Early ultrasounds measure the baby's size and compare it with standard growth charts to estimate gestational age.

• From that gestational age on the day of the scan, clinicians can back‑calculate an estimated LMP and due date.

First‑trimester ultrasounds are generally more accurate for dating than later scans, because early growth follows a tighter pattern, while later on there's more variation in size between babies.

If the ultrasound estimate is quite different from an LMP‑based due date, doctors may change the official due date to match the scan, using thresholds recommended in clinical guidelines.

Why do due dates differ between methods?

It's common to see slightly different dates from different methods. Reasons include:

• Cycle length and ovulation timing: If your cycles aren't 28 days or ovulation is earlier/later than day 14, the LMP method won't match the true conception date perfectly.

• Recall and recording: Not everyone remembers the exact first day of their period, and bleeding patterns can be irregular.

• Natural variation: Growth isn't identical for all pregnancies; some babies are a little smaller or larger at each stage, which can shift an ultrasound‑based estimate.

Because of this, clinicians treat due dates as a window rather than a fixed appointment; many healthy births happen across a range of weeks.

How calculators like SoftBump fit in

Online calculators bring these methods together:

• They let you enter different kinds of information — LMP date, a known conception or IVF transfer date, or an ultrasound date — and then apply the standard counting rules described above to estimate gestational age and a due date.

• They can help you understand how different methods relate to each other and why you might see slightly different numbers across apps, clinic letters or scans.

They are useful for understanding the timeline, but they don't replace your midwife or doctor in deciding how to schedule tests, interpret symptoms or manage the pregnancy.

A gentle reminder

If you are ever worried about your dates, your baby's movements, bleeding, pain or any other symptom, contact a healthcare professional urgently. No online calculator or blog, including this one, can safely diagnose or manage those situations.