🤰 4 Weeks Pregnant

You are in week 4 of your pregnancy — the first trimester, month 1. Your baby is about the size of a poppy seed. The blastocyst implants in the womb lining and starts making hCG, the hormone pregnancy tests detect.

By the Power Calculators team Updated 5 Nov 2026

Baby is the size ofa poppy seed

TrimesterFirst

MonthMonth 1

Weeks to due date36

Your baby at 4 weeks

The blastocyst implants in the womb lining and starts making hCG, the hormone pregnancy tests detect.
It splits into the cells that become the baby and those that become the placenta.

Average length	0.1–0.2 cm
Size comparison	A poppy seed
Days completed	28 of 280 (10%)

Your body at 4 weeks

Some women notice light spotting (implantation bleeding) or mild cramps.
Your period is due — a missed period is often the first sign.

To do this week

A home pregnancy test can turn positive from about the day your period is due.
Book your first antenatal appointment.

When to call your doctor

Vaginal bleeding or fluid leaking
Severe tummy pain or cramping
Fever over 38 °C
Severe headache, blurred vision or sudden swelling of face or hands
Vomiting so much you cannot keep fluids down

Pregnancy Due Date Calculator

Find your estimated due date, conception date and how far along you are, from the first day of your last period.

Pregnancy Week Calculator

Find exactly how many weeks and days pregnant you are today, your trimester and your due date.

When to Take a Pregnancy Test Calculator

Find the earliest day to take a pregnancy test and the most accurate day, from your last period and cycle length, with implantation dates and test accuracy by day.

Frequently asked questions

How many months is 4 weeks pregnant?

4 weeks is in month 1 of pregnancy as months are usually counted (about 0.9 calendar months since your last period), in the first trimester.

How big is the baby at 4 weeks?

About the size of a poppy seed.

What should I do at 4 weeks pregnant?

A home pregnancy test can turn positive from about the day your period is due. Book your first antenatal appointment.