Ultrasound: At some point during your pregnancy, you will likely have an ultrasound scan. This enables your healthcare provider to check on fetal growth and monitor other development milestones; it also provides the most accurate estimate for how many weeks pregnant you are and what your due date will be. This method is the most reliable, and particularly useful if you don't know the date of your last period or if your menstrual cycles are irregular. When figuring out how far along you are, it can be confusing that the weeks of pregnancy are not the same as the gestational age of the baby.
Remember, conception most likely occurred about 14 days into your last cycle, whereas pregnancy is calculated from the beginning of that cycle, so there's a two-week difference. For example, when you're six weeks pregnant, your baby's gestational age is only four weeks. During your prenatal checkups, your healthcare provider will monitor how far along you are.
Don't be surprised, though, if your provider gives you a slightly different due date than the one you calculated. Also, most babies don't arrive exactly on their due date. Every pregnancy is unique, and only a small percentage of babies are born right on time. Babies are typically born between weeks 38 and 42, and it's totally normal for your baby to arrive any time during this window.
If your baby is born pre-term , your doctor will be able to advise you on any extra care your baby might need. On the other hand, if your baby isn't born by the end of week 42, your doctor may suggest inducing the labor to bring on a vaginal birth , or he may discuss the option of a cesarean section with you.
Now that you know how far along you are, it might be helpful to know that pregnancies are often referred to in weeks and trimesters. Take a look at this pregnancy timeline for a breakdown of the weeks, months, and trimesters of pregnancy :. Here are some of the things you can expect in each trimester of pregnancy :. Ultrasound due date: If a due date has been calculated from an ultrasound in the first trimester, the reverse calculation method is preferred.
This calculates the current gestational age and gives an estimated date of LMP. Conception date: If conception is known eg, date of insemination, ovulation or egg retrieval , then the due date can be calculated by adding days. Gestation at a future or past date: Use to calculate gestation in the future or past. By far, the most common and accurate way to figure out your estimated due date is to take the start date of your last normal period and add days 40 weeks , which is the typical length of a pregnancy.
The formula for calculating how far along you are or number of weeks pregnant is simply the number of days since the first day of your last menstrual period LMP divided by seven:. It takes about nine days after ovulation for implantation to occur, which brings you to around day It takes a few days for your pregnancy hormone levels to be detectable by a pregnancy test, bringing you to around day A dating ultrasound , which measures the length of the fetus from crown to rump, is an accurate way to estimate your due date or how far along you are if completed within the first trimester the first 13 weeks of pregnancy.
Once you get past 13 weeks, the variability of the growth is such that an ultrasound can be off by a week or so. You can rely on the due date suggested by your last menstrual period, but your healthcare provider may schedule one anyway. Unfortunately, these tests are pretty vague, stating only one to two weeks, two to three weeks or three-plus weeks pregnant. In addition, the tests measure from implantation, not from your LMP. Historically, the first heartbeat and first movement are used to get a sense of how far along you are or the age of the fetus, but these forms of measurement are now considered outdated.
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