Pregnancy Week Calculator
Find out how many weeks pregnant you are, which trimester you are in, and track your pregnancy progress toward your due date.
Pregnancy Journey
First Trimester
Weeks 1–12
Second Trimester
Weeks 13–27
Third Trimester
Weeks 28–40
Full Term
Week 37+
Pregnancy lasts approximately 40 weeks
Enter Your Last Menstrual Period
Pregnancy is measured from the first day of your last period, not conception.
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How Pregnancy Weeks Are Counted
Pregnancy weeks are counted from the first day of the last menstrual period (LMP), not from the day of conception. This can seem counterintuitive — it means that when the egg is fertilised (approximately two weeks after the LMP in a 28-day cycle), you are already considered to be in week two of pregnancy. The reason for this convention is practical: the LMP date is almost always known precisely, while the exact date of ovulation or conception is often uncertain.
This system of dating is called gestational age, and it is used universally in obstetric care. Gestational age is different from embryonic age (or fetal age), which is counted from the date of fertilisation. Embryonic age is typically about two weeks less than gestational age. When doctors and midwives refer to how far along a pregnancy is, they always use gestational age unless stated otherwise.
Early ultrasound scans in the first trimester (ideally at 11–14 weeks) measure the size of the embryo or fetus and compare it to standard growth charts to confirm or revise the gestational age estimate. This is considered more accurate than LMP dating alone, particularly for people with irregular cycles.
What Happens Each Week of Pregnancy
In the first trimester (weeks 1–12), the most critical early development takes place. By week 4, the embryo implants in the uterine lining and begins producing hCG — the hormone detected by pregnancy tests. By week 6–7, a heartbeat may be detectable on ultrasound. By the end of the first trimester, all major organ systems have begun to form and the risk of miscarriage drops significantly.
The second trimester (weeks 13–27) is often called the 'honeymoon period' of pregnancy. Nausea typically improves, energy returns, and the pregnancy becomes visibly apparent. An anatomy scan between weeks 18 and 20 checks the baby's development in detail. By week 24, the pregnancy reaches the viability milestone — the point from which a baby born prematurely has a reasonable chance of survival with intensive care.
The third trimester (weeks 28–40) involves rapid weight gain for the baby and increasing preparation for birth. The baby's lungs mature, brain development accelerates, and the baby turns head-down in preparation for delivery. Week 37 marks full term — the point at which birth is considered safe without medical concerns about prematurity.
What Trimester Am I In?
Pregnancy is divided into three trimesters, each lasting approximately three months. The first trimester (weeks 1–12) covers early development and is often accompanied by morning sickness, fatigue, and heightened sensitivity to smells. Miscarriage risk is highest during this period, which is why many people wait until the end of the first trimester before widely announcing the pregnancy.
The second trimester (weeks 13–27) is typically the most comfortable period. The pregnancy bump becomes visible, fetal movements ('quickening') are felt for the first time, and the anatomy scan provides a detailed check of the baby's development. Many routine screening tests and genetic testing options occur during this trimester.
The third trimester (weeks 28–40) is characterised by physical discomfort as the baby grows and the uterus presses on surrounding organs. Antenatal appointments become more frequent, and preparations for birth begin in earnest. After week 37, the pregnancy is considered full term, and labour can begin at any time.
Frequently Asked Questions
How many weeks is a full pregnancy?
A full-term pregnancy is considered to last 40 weeks (280 days), measured from the first day of the last menstrual period (LMP). Most healthcare providers use this LMP-based gestational dating. Babies born between 37 and 42 weeks are considered full term. Only about 5% of babies are born on their exact estimated due date, but most births occur within two weeks of the predicted date in either direction.
How is pregnancy week calculated?
Pregnancy week is calculated from the first day of your last menstrual period, not from the date of conception or ovulation. This means that at the moment of conception (typically around day 14 of a 28-day cycle), you are already considered to be approximately 2 weeks pregnant by gestational age. This convention is used universally in obstetric care because the LMP date is typically known precisely, while conception date is not.
What trimester am I in?
Pregnancy is divided into three trimesters. The first trimester covers weeks 1 to 12 and includes the period of most rapid early development. The second trimester runs from week 13 to week 27 — most people find this the most comfortable phase, and it is when the anatomy scan is performed. The third trimester covers weeks 28 to 40, during which the baby gains weight and prepares for birth.
Can due dates change during pregnancy?
Yes — due dates are frequently revised after an early ultrasound, which is considered the most accurate method for dating a pregnancy in the first trimester. If the ultrasound measurements suggest a different gestational age from LMP calculations (by more than a week), the due date may be updated. This is normal and does not indicate a problem. Later ultrasounds are less reliable for due date estimation because individual growth variation between babies becomes larger.
