< WhenMyBaby Pregnancy Calculators...

Pregnancy Weeks Calendar

If you think (or know) you got pregnant this cycle, view your pregnancy weeks calendar based on your Last Menstrual Period (LMP).

Pregnant But Not Pregnant - Those First Two Weeks

Naegle's Rule, which uses the first Day of Last Menstrual Period (LMP) date, is the de facto standard for dating when pregnancy begins, even though we're not actually pregnant the first two weeks, given that it's technically impossible: LMP starts a brand new cycle while closing out the previous one: because pregnancy did not occur in the previous cycle, the uterine lining which had built up in preparation of pregnancy is shed, causing menstruation.

Pregnancy occurs when:

  1. You ovulate and the egg is fertilized by a sperm. An amazing feat for our tiny egg, which only survives for about 1 day, during which time she needs to hook up with one sperm from among millions!.
  2. The fused egg and sperm, called a zygote, implants into your uterine lining

As a result, since ovulation occurs around mid-cycle and implantation generally occurs about 7 to 10 days after fertility, women are not actually pregnant the first two weeks or so of pregnancy!

Your cycle, as are other women's, is unique, and there are several factors that determine when in your cycle you will ovulate, and then how long until implantation occurs. Compound that with the fact that few women have the perfect 28 day cycle and 14 day luteal phase, making it very difficult to date pregnancy based on ovulation. For that reason, LMP is the standard, because when our last period started is a date we all tend to remember, facilitated in the modern age with trackers like MyMonthlyCycles (born 2003).

Why are most babies born early or late?

There's a good reason that only about 5% of babies are born on their estimated due date; In large part, it's because LMP is used to date pregnancy.

If all women always had a 28 day cycle, and a 14 day luteal phase, every woman would ovulate very close to cycle day 14. Obviously, one would still not be pregnant for the first two weeks from LMP. Due dates would still be off from EDD, but only by a few days, since time to implantat is still a variable.

The fact is most women do not have a 28 day cycle with 14 day luteal. That makes all the difference and is why so many babies are not born on their EDD: for women with shorter cycles and/or longer luteal phases, they may be truly pregnant earlier in their cycle; for those with longer cycles and/or shorter luteals, they may not be actually pregnant for 3 to 4 weeks, or more, after LMP.

Imiportant Note: Ultrasound is also used to date pregnancy. First trimester ultrasound has an accuracy of of +/- 5 to 7 days, as compared to Naegle's Rule which is +/- 14 days. [Source: Pregnancy Dating, Authors John A. Morgan; Danielle B. Cooper, at www.ncbi.nlm.nih.gov]. However, early ultrasound, if done, is usually performed at 6 to 8 weeks of pregnancy; up until then, Naegle's Rule rules when it comes to dating early pregnancy!