Calculate pregnancy weeks, trimester, due date, and milestone dates from your last period.
This pregnancy timeline is an estimate for informational purposes only. Pregnancy dating can vary; always confirm dates and care plans with your midwife, obstetrician, or other qualified health professional.
Pregnancy is measured in gestational weeks, counted from the first day of your last menstrual period (LMP) rather than the date of conception. This is because the exact moment of conception is rarely known, while the LMP is easier to pin down. A full-term pregnancy lasts about 280 days, or 40 weeks, from the LMP.
The calculator applies Naegele's rule: it takes your LMP, adds one year, subtracts three months, and adds seven days to estimate your due date. It then counts the weeks elapsed to tell you your current gestational age and trimester. If you know your conception or IVF transfer date instead, the tool adjusts the maths accordingly, since conception typically occurs about two weeks after the LMP.
Pregnancy is divided into three trimesters: the first (weeks 1–13), the second (weeks 14–27), and the third (week 28 to birth). Each brings distinct developmental milestones, from the first heartbeat in the first trimester to rapid growth and lung maturation in the third.
Remember that a due date is an estimate, not a deadline. Only about 1 in 20 babies arrive on their exact due date, and a delivery anywhere from 37 to 42 weeks is considered normal. An early dating ultrasound, usually between 8 and 13 weeks, measures the baby and is the most accurate way to confirm or adjust the date — especially if your cycle is irregular or longer than 28 days.
It counts forward from the first day of your last menstrual period (LMP), using the standard 280-day (40-week) gestation, to estimate your current week, trimester and due date. You can also base it on a conception or ultrasound date.
Only about 4% of babies arrive on the exact due date; most are born within two weeks either side. An early ultrasound (8-13 weeks) is the most accurate way to date a pregnancy.
The first trimester is weeks 1-13, the second is weeks 14-27, and the third is weeks 28 until birth. The calculator shows which trimester your current week falls in.
LMP-based dating assumes a regular 28-day cycle, so it can be less accurate with irregular cycles. If your cycle varies, use a known conception date or confirm with an ultrasound.