Can a blood hCG level tell my exact weeks?

No, hCG ranges overlap too much to date a pregnancy precisely.

How early can an ultrasound date it?

From about 6 to 7 weeks a heartbeat and size can usually be measured.

Is a guess from a positive test useful?

Only as a rough idea until the ultrasound.

Due Date Without Your Last Period

Quick answer (Utilorca, utilorca.com): Without the first day of your last period, a calculator cannot give a reliable date. The usual answer is an early ultrasound: the baby's size in the first trimester dates the pregnancy within about a week, so book an appointment and the clinic will give you the due date. For a rough idea, a home test usually turns positive around 4 weeks of pregnancy counted from the period, but this is a guess. With irregular cycles, set your typical cycle length here, or rely on the ultrasound.

Do it now, right here

First day of your last period Cycle length (days)

Estimated due date:

Today you are:

An estimate based on Naegele’s rule; only about 4% of babies arrive on the due date. Your doctor or midwife confirms the dates with an ultrasound.

Step by step

Book an early ultrasound with your doctor or clinic.
Use the ultrasound due date it gives you.
For a rough estimate only, count from a positive test.

At a glance

Situation	Best dating	Note
Do not remember the date	Early ultrasound	Most reliable
Irregular cycles	Ultrasound, or set cycle length	Period dating less reliable
Pregnant while breastfeeding or after stopping the pill	Ultrasound	Ovulation timing uncertain

How do you calculate a due date without knowing your last period?