How Many Weeks Pregnant Am I? A Simple Guide to Dating Your Pregnancy

How many weeks pregnant am I — quick answers

How do I calculate how many weeks pregnant I am?

Count from the first day of your last menstrual period (LMP). If your last period started 8 weeks ago, you are 8 weeks pregnant, even though conception happened about 2 weeks after that date.

How is the due date calculated?

Add 280 days (40 weeks) to the first day of your last period. The quick method is Naegele's rule: take the LMP date, add 7 days, then add 9 months.

Why does the scan give a different date?

An early ultrasound measures the baby directly and is accurate to within about 5 days. If it differs from your period date by more than a week, doctors use the scan date, because ovulation does not always happen on day 14.

How many weeks is a full-term pregnancy?

39 to 40 weeks is full term. Babies born between 37 and 38 weeks are "early term", and before 37 weeks is preterm.

How many weeks are in each trimester?

First trimester: weeks 1 to 12. Second trimester: weeks 13 to 27. Third trimester: week 28 until birth.

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One of the first questions after a positive test is "how far along am I?" The answer is simpler than it seems once you know the rule doctors use, and knowing your dates matters, because every scan, blood test and milestone in pregnancy is timed by the week.

The pregnancy is dated from your last period, not from conception

It surprises many women that they are "4 weeks pregnant" on the day the test turns positive, when conception only happened about two weeks earlier. Pregnancy is counted in gestational weeks from the first day of the last menstrual period (LMP), because that is a date most women know, whereas the day of ovulation and conception usually is not.

So in a typical 28-day cycle:

• Week 1 and 2: your period and the lead-up to ovulation. You are not yet pregnant.
• Week 2 to 3: ovulation and fertilisation.
• Week 4: the embryo implants and a home pregnancy test becomes positive.
• Week 5 to 6: a heartbeat can first be seen on ultrasound.

Working out your weeks and due date

Step 1. Note the first day of your last period.

Step 2. Count the weeks from that day to today. That is your gestational age, written as weeks plus days, for example "9 weeks 3 days".

Step 3. For the due date, add 7 days to the LMP date and then add 9 months (Naegele's rule). Last period started 10 January: add 7 days gives 17 January, add 9 months gives 17 October.

This assumes a 28-day cycle with ovulation on day 14. If your cycles are longer, ovulation is later and the true date is a little later too; if they are shorter, it is a little earlier.

When the dates are uncertain

The LMP method does not work well if:

• your periods are irregular or you have PCOS,
• you were breastfeeding or recently stopped the pill,
• you do not remember the date, or
• you conceived through IVF (then the transfer date is used).

In these cases the dating scan decides. Between 7 and 13 weeks the sonographer measures the crown-rump length (CRL) of the embryo, which grows so predictably that the age can be fixed to within 3 to 5 days. After 14 weeks the head, abdomen and thigh bone are measured instead, which is less precise, so an early scan is the most reliable way to date a pregnancy.

If the scan and your period date disagree by more than about 7 days in the first trimester, the scan date replaces the period date and stays as your official due date for the rest of the pregnancy. Later scans are not used to change the due date, even if the baby measures big or small; those measurements are about growth, not age.

What the weeks mean for your care

Knowing the exact week lets your obstetrician time the important checks:

Weeks	What happens
6 to 9	Booking visit, blood tests, early scan to confirm dates and heartbeat
11 to 13	Nuchal translucency scan and first-trimester screening
18 to 22	Anomaly (structural) scan
24 to 28	Glucose test for gestational diabetes; anti-D if Rh negative
28 to 36	Growth scans if needed, tetanus and whooping-cough vaccines
36 onwards	Position of the baby, birth planning

Only about 5 percent of babies arrive on the due date

The due date is an estimate of the middle of a normal window, not a deadline. Most babies are born between 38 and 42 weeks. Your obstetrician will discuss induction if the pregnancy goes past 41 weeks.

If you have just found out you are pregnant, book an early antenatal visit. At Iswarya Hospital in OMR, Chennai, our obstetric team confirms your dates with an early scan and plans your care week by week from there.