Pregnancy Calculator: How Many Weeks Pregnant Am I?

Our pregnancy calculator answers the question “how many weeks pregnant am I?” Enter the first day of your last period, and it shows how far along you are in weeks and days, which trimester you are in, roughly how many months that is, and your estimated due date.

How the pregnancy calculator works

Doctors in Singapore and around the world count pregnancy from the first day of your last menstrual period (LMP), not from the day you conceived. A full-term pregnancy is about 40 weeks, or 280 days, from that date.

Because ovulation usually happens about two weeks after your period starts, you are already counted as around two weeks pregnant when you conceive, and around four weeks pregnant when your period is due. That surprises many people, but it is simply how pregnancy is dated.

The calculator assumes a regular 28-day cycle. If your cycles are usually longer or shorter, you may be a little further behind or ahead, and your dating scan will give a more accurate answer. To work out your due date or estimate Baby Bonus support, use the calculators on our tools page.

Trimesters at a glance

• First trimester: weeks 1 to 13. Your baby’s organs form, and nausea and tiredness are common. See our first trimester guide.
• Second trimester: weeks 14 to 27. Many mums feel more energetic and start to feel movements. See our second trimester guide.
• Third trimester: week 28 until birth. Your baby grows quickly and checkups become more frequent. See our third trimester guide.

Pregnancy weeks to months conversion

Months are an awkward way to measure pregnancy, because most calendar months are a little over four weeks long. That is why 40 weeks is often described as “nine months” even though it is closer to nine and a quarter. The table shows two ways to think about it: roughly how many months have passed since your last period, and which month of pregnancy you are in, using the common nine-month split.

So, for example, 20 weeks pregnant is about four and a half months, and you are in your fifth month. 12 weeks is close to three months, the end of the first trimester. Different books split the months slightly differently, so treat this as a guide.

How accurate is my due date?

Your due date is an estimate. Only a small minority of babies, roughly one in 20, arrive on the exact day; most are born between 37 and 42 weeks. Dates based on your last period can also be off if your cycles are irregular, you are not sure of your period date, or you conceived soon after stopping the pill or after a previous pregnancy.

A first-trimester dating scan is the most accurate way to date a pregnancy. Measuring the baby from head to bottom (the crown-rump length) in the first trimester is usually accurate to within about a week. If the scan and your period dates differ by more than a few days, your doctor will usually go by the scan. Scans later in pregnancy are less precise for dating, because babies grow at different rates. Our guide to pregnancy scans and tests in Singapore explains when each scan happens.