Month 1: Mystery of due dates
Babies don’t arrive on time, most of the time, and instead of talking about an ‘expected due date’, we should perhaps be talking about an ‘expected due period’.
Very few women can say with certainty that they know exactly when they fell pregnant, so calculating your baby’s estimated day of arrival can be very confusing.
When we talk about pregnancy length, we talk about trimesters (meaning three months). A normal pregnancy lasts about nine calendar months, which means there will be three trimesters. Some months have more days than others. Pregnancy length is often described in weeks. A normal pregnancy is considered to be 40 weeks long. But doesn’t that work out to be 10 months? When we date a pregnancy according to weeks, we talk about lunar months (28 days in a month), not calendar months.
In other words, 40 weeks works out to be nine calendar months of 30 or 31 days each, or 10 lunar months of 28 days each. To add to the confusion – pregnancy length is also described in days! You’re pregnant (in a normal full-term pregnancy) between 266 and 280 days. If you count your pregnancy as 280 days, you’re working on the dates of your LMP – last menstrual period – adding in two weeks of the “unknown” date of conception. When we date a pregnancy as 266 days, it has been calculated from the probable (known) date of conception.
SO HOW SHOULD I WORK IT OUT?
The most common method of dating a pregnancy is calculated based on the first day of your last period, which most women can remember – especially if you’re actively trying to fall pregnant. This method works quite well for women whose menstrual cycles last around 28 days. Most medical professionals prefer to use this method.
Another method of calculating your due date, known as Naegele’s Rule, is to take the first day of your last period, count backward three months, add seven days, and add one year. (Alternatively, take the date of your LMP, add one week, and then add nine months.)
The due date calculation works best if your menstrual cycle is regular and your periods are every 28 days.
If your periods are irregular, and you don’t remember the date of your LMP, and your menstrual cycle varies in length, it may be difficult to get a fairly accurate due date.
Some women get even more confused when they think they’ve had a “short, light period” only to discover they’re further along in their pregnancy than they thought. This is bleeding that may occur when the conceptus imbeds into the lining of the uterus, securing the pregnancy in the womb.
DON’T MISTAKE SPOTTING FOR YOUR PERIOD
When this happens, the tissue that forms around the egg may damage some of your own blood vessels in the uterus, resulting in a small amount of blood leaking from the cervix down the vagina.
This is often referred to as “spotting”. The bleeding usually occurs a few days before the menstrual period is due to start (10 to 14 days after conception) and can easily be mistaken for a period. Since implantation bleeding may be confused with the normal menstrual cycle, some women are surprised to discover their pregnancy is further along than they originally thought. This can cause confusion when trying to work out a due date of delivery.
Once the pregnancy is confirmed at a doctor’s office, they can use other testing to determine the correct gestational age of the foetus, particularly if implantation bleeding has left some question about when the last proper menstrual cycle occurred.
BOOK YOUR DATING SCAN
Few babies arrive on a due date. Some arrive too soon (before 37 weeks is considered premature), and the reason for this is not always clear. Other babies go beyond their due date, up to two weeks after they’re expected.
If you suspect that you may be pregnant, visit your doctor/clinic as soon as you can get an appointment. The first ultrasound scan you have will give you a more accurate idea of how far along you may be. This scan measures certain parts of your baby (crown to rump) and is known as the “dating scan”. This date tends to be more accurate than the date of later scans.