However, conception doesn't actually occur until around 14 days after your LMP, or later if your cycle is longer than 28 days. Multiple pregnancy: antenatal care for twin and triplet pregnancies CG129. You may know when your LMP was, but not when you actually conceived.

In the first case-based class of medical school, students are asked to answer a virtual patient’s question about the development of the fetus. It’s a great example of how detailed facts need the larger context.

These students are smart and they know all about beta Hc G and are anxious to showcase their knowledge of the menstrual cycle with fluctuating levels of various hormones (FSH, progesterone, etc.). The usual dating is gestational age, based on the first day of your last menstrual period.

The scan will usually show that your pregnancy is less far along than suggested by your LMP.

This means that accurate dating reduces your chance of having your labour induced because your baby is thought to be overdue.

Yet one question brings confusion, “How pregnant is this women? However, you can also date a pregnancy with embryological age, starting at conception.

” The related question, “When does pregnancy start? This graphic is intentionally simple, removing all the hormones and other fun stuff.

If you do know the conception date, this is counted as day 14, not day one.

If your cycle is irregular or you've recently been on the pill, working out the date from your LMP probably won't work. If the scan date differs from your period dates, the scan date will be used.

The point of the scan is to work out how many weeks pregnant you are. Ectopic pregnancy and miscarriage: diagnosis and initial management CG 154.