I was discussing unborn babies with a young person the other day (who does not have children yet) and they thought the human/baby heart started beating at about 8 months old. And that they weren't really considered a person until they were born. Also that they did not feel pain.
The fetus heart begins beating around the 5th week of gestation and is usually VISIBLY beating on a standard ultrasound at 6 weeks of gestation. 5 WEEKS. You can see the blood moving on the screen. We were not discussing the issue of abortion, but this person seemed to think (based on his guesses about how babies develop) that they didn't feel and didn't have heartbeats and bodily systems running until much closer to their birth date (his thoughts actually were that it was ok for women to drink because he thought the alcohol didn't affect the baby because it wasn't alive until about 8 months).
The date the woman should have gotten her period marks 4 weeks of gestation, so if she is 1 week late for her period, the baby is at 5 weeks of gestation already and it's heart is developing and almost ready to start beating. If she is 2 weeks late, the heart is already beating steadily along.
The second trimester runs from about week 13 to week 26. It is the third trimester after week 26. Women can generally feel the baby kicking and moving around week 18-20. After week 20, movement can generally be seen and felt from outside the woman's body by others.
The earliest viability for a fetus outside the womb is currently 23-24 weeks depending on the hospital and the baby. Any baby born that early, however, will face a long and scary NICU stay and probably several health issues for the first several months of life, but there are many examples of babies born at early gestational ages that are quite happy and healthy. It is not an easy road for the parents or the children, but it is a road that can be taken, thanks to advanced medicine.
Any way, it was brought to my attention that some people (so far I'm finding it's younger people that haven't even begun to enter the 'wanting kids' stage of life. I was this way once! A long time ago!) were unaware of some of the finer points of human development at these early stages and I did not want anyone going on believing when the fetuses began to move, have heartbeats, be affected by alcohol/drugs the mother was ingesting, etc.