Dating ultrasound measurements
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. Ultrasound scans are considered to be safe for you and your baby when performed by trained professionals. However, sometimes, having a scan can cause unnecessary worry. It's entirely up to you whether or not you have a scan. You could have a dating scan without NT screening, or even change your mind on the day of your scan. It may also help you to avoid feeling disappointed towards the end of your pregnancy if you think your baby is late when he's not. The scan can check that your baby's heart is beating nicely and that he's developing normally.
Therefore, from 13 weeks, or if your baby is longer than 84mm, the circumference of the head becomes the best way to measure your baby. Evaluation of gestation emedicine.[Accessed July 2017] Newson L. Most units offer a screening test for Down's syndrome and other conditions at the same time as the dating scan. Screening for Down's syndrome involves a nuchal translucency (NT) scan plus a blood test. You can have the combined screening test from 11 weeks to 13 weeks plus six days, or when your baby's CRL is 45mm (1.8in) to 84mm (3.3in). A blood test at what you thought was 13 weeks wouldn't be helpful if you were actually 16 weeks pregnant. It may lead to you being given a high chance for Down's syndrome, when your baby is actually developing normally (false-positive result). Addition of non-invasive test to improve screening for pregnant women. 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. It shouldn't be uncomfortable, and you don't need a full bladder Your sonographer will give you a report of the scan, which will tell you exactly how many weeks pregnant you are. Keep it with your maternity notes so your midwife and anyone else involved in your care can see it. This measurement, known as the crown rump length (CRL), is very accurate in the first trimester. It won't hurt, although you may feel a little pressure on your tummy. If your womb (uterus) is very deep in your pelvis, or if you're overweight, you may be offered a vaginal scan, which can get a closer view of your baby.