Calculating the week of pregnancy
Why knowing the exact week of pregnancy matters clinically, why weeks are used rather than months, and how the date is calculated.
Why it matters
Establishing gestational age begins with the question asked at antenatal appointments: when was your last period? From that point the estimated date of delivery is set and good antenatal care can begin.
Human pregnancy lasts 280 days — counted from the first day of the last menstrual period to delivery — with a standard deviation of 14 days. The week of pregnancy is calculated from the first day of the last period, not from the moment of conception. Clinically, then, pregnancy corresponds to a span of about 40 weeks.
Confusion arises mostly among those who think of pregnancy as nine months (it is in fact 40 ÷ 4 = ten months) or who count from conception. Another difficulty arises in women whose cycle is longer than 28 days, because fertilisation did not occur on day 14.
A commonly used phrase such as "four months pregnant" — is that 16 weeks or 20? — has no clear meaning and is no longer used in antenatal records. When speaking to families about the length of a pregnancy it is more useful to use weeks rather than months, and to divide pregnancy into three parts: 1-14 weeks, 14-28 weeks, and 28 weeks to delivery (the first, second and third trimesters).
Knowing the week of pregnancy is critical to the decisions a doctor makes. In a normal pregnancy the weight of the foetus can be estimated from gestational age, and those two factors are important throughout pregnancy in judging the foetus's wellbeing. Without knowing gestational age it is often not possible to diagnose a post-term pregnancy or restricted growth in the womb. Knowing gestational age and the baby's weight is essential to managing a premature birth or a problematic pregnancy appropriately. Accurate recording of gestational age in hospitals matters for the flow of information, and rapid access to it is vital. In certain clinical situations, or where a long hospital stay is needed, it is more useful to note the pregnancy in weeks and days (27+4/7 weeks).
How it is calculated
Accurate information about the last menstrual period is the most reliable clinical method for estimating gestational age. Using Naegele's rule, the due date is found by subtracting three months from the first day of the last period and adding one week.
If the date of the last period is not known, the most reliable method is an ultrasound examination in the early weeks of pregnancy (the first 13-14 weeks). This allows the week of pregnancy to be calculated with a margin of error of about one week.
See also the pregnancy week calculator.