How Far Along Would I Be If I Conceived?

If you know roughly when you conceived, you can count from there — but the number a clinician will use is about two weeks larger. Enter the estimated conception date to see both: weeks since fertilization, and the equivalent gestational age.

Two counts, two weeks apart

MedlinePlus defines gestational age as the measure of how far along a pregnancy is, counted in weeks from the first day of the last menstrual cycle to the current date. Fertilization happens around two weeks after that day, in the middle of a typical cycle.

So the two counts never agree:

Weeks since conception	Gestational age	Common description
2	4 weeks	The week a period would be missed
4	6 weeks	Early first trimester
6	8 weeks	Often the first prenatal visit
10	12 weeks	End of the first trimester
26	28 weeks	Start of the third trimester

At the instant of conception a pregnancy is already dated at about two weeks. It is why “40 weeks” contains roughly 38 weeks of an actual embryo and fetus, and why the months you count from a positive test never line up with the chart — a gap worked through in pregnancy weeks to months.

Worked example

Conception on March 1, today is April 12.

• Days since conception: 42
• Weeks since conception: 6
• Equivalent gestational age: about 8 weeks

Your conception date is an estimate too

Even people tracking ovulation are working with a window rather than a moment. A released egg survives less than 24 hours, sperm survive up to five days inside the body, and the highest pregnancy rates are reported when the two meet within 4 to 6 hours of ovulation. An act of intercourse and the fertilization it caused can therefore be several days apart, which puts a few days of uncertainty into any conception date before the arithmetic even starts.

That uncertainty is the practical argument for the clinical convention. A last period is a date somebody actually observed; a conception date is usually reconstructed.

The scan outranks the calendar

Before birth, gestational age is estimated by ultrasound measurements of the head, abdomen and thigh bone. An early scan measures the fetus directly and can move a date derived from a remembered period — which is why the number changes sometimes, and why the ultrasound version is the one that ends up in the chart.

The reference points that follow from it are all gestational: a normal pregnancy runs 38 to 42 weeks, before 37 weeks is premature, after 42 is postmature.

If the pregnancy is not confirmed yet, start at when to take a pregnancy test; if it is, the hormone your clinician may follow in these first weeks is covered in hCG levels by week.

This content is informational and does not replace prenatal care, which is where a pregnancy is dated for real.