Baby’s Health Center
Calculating the day your baby begins to develop and keeping track of your pregnancy dates can be a challenge. The development of pregnancy is counted from the first day of the woman’s last normal period, even though the development of the fetus does not begin until conception. Pregnancy is calculated from this day because each time a woman has a period, her body is preparing for pregnancy.
The following information is used as a general guide for healthy pregnancy development, although development may vary due to the mother’s health or a miscalculation of ovulation. Gestational age is the age of the pregnancy from the last normal menstrual period (LMP), and fetal age is the actual age of the growing baby. Most references to pregnancy are usually in gestational age rather than fetal age development, but we have included both so that it is clear what stage development is at. Measurements will be given in total length from head to toe, but each pregnancy can differ in weight and length measurements, and these are just a general guideline.
Pregnancy is also divided into trimesters which last about 12 – 14 weeks each. Similar to development, these can be calculated from different dates so not all trimester calculations will equal the same. The following information divides the three trimesters into a little over 3 completed months each. The first trimester is week 1 through the end of week 13. The second trimester usually ends around the 26th week and consists of the 4th, 5th and 6th completed months. The third trimester can end anywhere between the 38th – 42nd week and is the 7th, 8th and 9th completed months of pregnancy.