4 things to know about due dates in pregnancy:
- Pregnancy is dated using an ultrasound scan done at around 11-13 weeks
- Studies have shown that dating using ultrasound is more accurate that period dates
- Dating by ultrasound scan in later pregnancy is much less accurate.
- The end of a pregnancy is considered to be between 37 and 42 weeks
How are dates initially calculated?
When you attend your first antenatal clinic appointment, the midwife will ask you for the first date of your last period, and will use this date to estimate how many weeks pregnant you are. This information will be used to arrange your first ultrasound scan.
Your first scan will usually be planned for around 11-13 weeks. This will measure the baby from the top of the head to the bottom, a measurement which is unattractively called the crown rump length (CRL). This measurement is used to calculate the date that the baby will be 40 weeks. This date is then known as your estimated due date (EDD for short). It is not very likely that the baby will be born on this date (most babies are not born on the due date). It is mainly used in the pregnancy to calculate how many weeks you are when you attend the hospital.
Why do we use the ultrasound scan dates rather that my period dates?
The simple answer to this is that ultrasound dates, as long as they are measured early, are more accurate than period dates. This has been shown in studies.
Period dates are based on everyone having an exactly 28 day cycle and conceiving exactly on day 14, which is not often the case!! Also, the day you have sex might not be the day that conception occurs because the sperm can survive for up to 48 hours.
Ultrasound scans become more inaccurate for dating later in the pregnancy. This is particularly the case after 16-20 weeks. So, do not try to re-calculate an estimated due date based on later measurements in the pregnancy, because this will be very inaccurate.
How long should be pregnancy be in total?
Even though your midwife and doctor will use a specific date to work from during the pregnancy, we consider the ‘end’ of pregnancy, called ‘term’ to be anything between 37 and 42 weeks. Less than 37 weeks is considered pre-term, or premature.Â
What if my pregnancy carries on longer than expected?
Usually, in the UK, if you are having a planned caesarean section, the aim would be to do this procedure after 39 weeks. If you do go overdue, which happens to quite a few women, then you will usually be offered induction of labour after 41 weeks.
Discover more from Expert Pregnancy Safety Guidance & Maternity Advice | The Authentic Pregnancy Doctor
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