Dating a pregnancy
) and as the reality of that pink line starts to set in, you reach for your calendar to mark down the big D-day (due date). You know which day you started bleeding, but do you know with 100 percent accuracy the day you ovulated?
In fact, a full-term pregnancy is considered to be anywhere from 38 weeks to 42 weeks long (a baby born at 38 weeks isn't "early" anymore than one born at 42 weeks is "late"). It seems odd that the medical powers that be (and that determine these kinds of things) would start the time clock on pregnancy before sperm even meets egg (and before your ovary even releases the egg), but it actually makes perfect sense.
The timing of certain tests, the monitoring of the baby's growth, and the correct diagnosis of premature labor, or being truly "overdue," (postdates), as well as many other situations that arise in the course of a typical pregnancy, all depend on a correct determination of the EDC for appropriate management.
In the past, the EDC was calculated by using Naegele's Rule, which determined the date by subtracting 3 months from the 1st day of the last period and then adding 7 days.
Pregnancy lasts an average of 280 days (40 weeks) from the first day of your last menstrual period (LMP).
The first day of your LMP is considered day 1 of pregnancy, even though you probably didn't conceive until about two weeks later (fetal development lags two weeks behind your pregnancy dates).
With today's modern equipment, we can obtain very reliable images and measurements of even very early pregnancies, sometimes even seeing a heartbeat as early as 5-6 weeks!
The most common misconception we encounter almost daily, has to do with how accurate ultrasounds are in fixing the EDC at different stages of pregnancy.
Some women have cycles that are consistently longer than the average 28-day cycle.
And even if you do (thanks to ovulation testers, basal body thermometers, and cervical mucus changes), do you know with absolute certainty the date you conceived?
(Remember, an egg can be fertilized for 24 hours — or possibly longer — after ovulation occurs.
As a matter of convenience (because you need idea of when your baby will arrive) and convention (because it's important to have developmental benchmarks to measure your baby against), a pregnancy is calculated as 40 weeks long — even though only about five percent of babies actually stay in utero 40 weeks to the day. To make things a bit more complicated, the 40 weeks of pregnancy are not counted from the day (or passionate night) you conceived — they're counted from the first day of your last menstrual period (presumably, not so passionate).
You've only been pregnant five minutes, and already you're confused. Take a seat, a deep breath, and get ready for pregnancy math 101 (and don't worry — you don't need to pull out your high school algebra notes…or worry if you didn't actually get around to taking any).