Pregnancy Week by Week

Though pregnancy is most commonly divided into trimesters or months, following your pregnancy week by week can help you get a better sense of your baby's development and the changes going on within your own body. Fetal development happens very rapidly, and pregnancy symptoms can vary just as quickly. Following week by week allows you to mark these changes more accurately than if you were tracking month by month or trimester by trimester.

Accurately tracking your pregnancy can not only be interesting but may also be incredibly important for the safety of both you and your fetus. What might be a normal pregnancy symptom at one stage may be abnormal at another. For example, cramping and bleeding early on, especially around the time of implantation (one to two weeks after conception) is fairly normal, whereas later in pregnancy, it can be a sign of trouble.

Knowing what to expect in your pregnancy week 1 all the way to week 40 will help ensure that you recognize the danger signs early, and, on the lighter side, will help you engage with your developing baby and enjoy the experience of being pregnant.

Gestational vs. Fetal Age

One very important concept in week by week pregnancy tracking is the difference between fetal age and gestational age. Because it is nearly impossible to pinpoint the exact moment of conception, pregnancy is often dated from the first day of your last normal menstrual period (LNMP, or simply LMP). In other words, the first week of pregnancy is actually the week of your last period, and conception happens somewhere around week two. So, according to gestational age, your baby starts at about two weeks old.

Fetal age, on the other hand, is counted from the approximate date of conception. Because it is inexact, it is often compared with gestational age in order to get a better idea of how fetal development is progressing and to more accurately estimate a due date.

Week by Week Pregnancy Guide

Though you can find more detail on each individual week of pregnancy, here are some milestone weeks to note (estimated according to gestational age):

  • Week 1 - Last menstrual period
  • Week 2 - Fertilization; conception (zygote)
  • Week 3 - Zygote becomes blastocyst
  • Week 4 - Implantation; blastocyst becomes embryo
  • Week 8 - Embryo becomes fetus
  • Week 12 - End of first trimester; danger of miscarriage declines markedly
  • Week 13 - Second trimester begins
  • Week 22 - Sex of fetus can be determined by ultrasound (usually)
  • Week 28 - Third trimester begins
  • Week 35 - Period of rapid fetal weight gain begins
  • Week 37 - Fetus is considered full-term
  • Week 40 - Baby should be born around now, if s/he hasn't arrived already
  • Week 42 - Baby is officially overdue; labor may be induced

Pregnancy Timeline