The Fetal Development information is used as a general guide for healthy pregnancy development, this 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 given are a total length from crown to rump (CRL) or [head to buttucks] up to week 20, and after week 20 they are given as head to heel. Each pregnancy can differ in weight and length measurements, and those given are just a general guideline.
Pregnancy is also divided into trimesters which last about 12 - 14 weeks each (depending on source). Similar to development, these can be calculated from different dates so not all trimester calculations will equal the same. The information here divides the three trimesters into 3 completed months each. The first trimester is week 1 through to 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.