Congratulations, you think (or know) you might be pregnant! But bear in mind it may be quite confusing to try to work out how many weeks pregnant you are.
That’s because of the way that doctors and midwives have traditionally calculated the ‘weeks pregnant’ timeframe, with 40 weeks being the due date.
Doctors and midwives generally discuss pregnancies in weeks, rather than months, so they can more accurately assess the growth and development of the baby throughout the pregnancy.
The average pregnancy is assumed to be a total of 40 weeks (that’s 280 days) from the first day of your last menstrual period (often shortened to ‘LMP.’) But anytime between 30-40 weeks is still considered “term”.
Pregnancy is commonly referred to in terms of ‘gestational age’ rather than ‘foetal age development.’ When you think about it carefully, that means that the average gestation only goes for 38 weeks after fertilisation – and using this system, you are already two weeks ‘pregnant’ at the time of ovulation!
To work out how many weeks pregnant you are under the traditional system (which is still used in most hospitals in Australia and New Zealand), just work forward from the first day of your last menstrual period.
How many weeks pregnant am I – under the foetal age (or ovulation) system?
Sometimes doctors and midwives talk about the ‘foetal age’ or use the ‘ovulation’ system of determining pregnancy weeks. This is because these measurements are generally more accurate than assuming ovulation has occurred 14 days after the first day of the last period.
The foetal age is the actual age of your baby – that is, how many weeks since the egg and sperm joined together in conception.
Thanks to technologies like ultrasound, as well as widely-available and increasingly accurate ovulation testing, foetal age can be determined much more easily and give a better estimate of actual pregnancy weeks.
If you have a shorter or longer menstrual cycle than the standard 28 days, using the standard gestational age ‘dates’ won’t quite match up with your baby’s gestation, so foetal age can be more accurate.
Pregnancy is usually divided into three ‘trimesters’ of around twelve weeks each. That’s because these represent three quite different stages of pregnancy in terms of the experiences of a pregnant woman and her baby’s development.
How many weeks will your pregnancy be?
The main reason that most women want to know how many weeks pregnant they are, is because they want to know what date their baby will be born. This is also known as the expected date of confinement (EDC) or expected date of delivery (EDD).
The traditional method of estimating the due date of a baby, called Naegele’s rule, is to add one year then take away three months and add seven days to the first day of the last normal period.
So, if your last period started on 18 November, then your due date is going to be the 25 August, which works out to around 40 weeks. Naegele’s rule assumes the woman has a 28 day menstrual cycle and ovulated and fell pregnant on day 14; so gestation is 38 weeks from the day of ovulation.
However, ovulation and conception are not that simple. Around only 5% of babies actually arrive on their due date, much to the frustration of most expectant parents and waiting families.
Statistically first time mothers are more likely to be overdue than have their baby before the due date.
If the pregnancy has been healthy and without complications, most babies are ready to be born between 38-42 weeks.
Current statistics show that around 8% of babies are born premature, i.e. before 37 weeks of gestation.
By Fran Molloy, journalist and mum of four
Last Published* November, 2021
*Please note that the published date may not be the same as the date that the content was created and that information above may have changed since.