Let’s Dish on Dates: Last Menstrual Period (LMP)

When most of us hear the term dating, we think back to those days when we had butterflies in our stomach every time we imagined meeting our plus one for dinner and drinks. Questions from what should I wear to what should I say to when is it okay to stay flooded our minds. Fast-forward a few years, and now flings are out and fertility is in. Dates are now dictated by the arrival of our period and the most promising days to have sex.

The calendar is littered with red Xs and black circles rather than dinner reservations and drink locations. And while your chicken scratches can start to look like hieroglyphics, here’s why those “X marks the spot” notations really matter!

Pregnancies are dated (a.k.a. the due date of a pregnancy is calculated) based on the first day of a woman’s last menstrual period (LMP). Your chart will refer to this date as your EDC, a precise 40 weeks from your LMP.  This date is used to calculate when you ovulated and, therefore, when your egg met sperm. To confirm that you are spot on with when you saw that first “spot,” your OB/GYN will perform an early ultrasound measuring the length of the fetus (medically termed the crown rump length) to confirm that your date is consistent with the dates being picked up on the ultrasound. The two need to jive for the due date to be written in stone (or at least in your medical chart in black marker!).

In cases where there is a serious discrepancy, your OB will often re-date the pregnancy (that is, calculate a new due date based on the measurements noted on the ultrasound). Re-dating is dependent on how pregnant you are measuring at the time of the ultrasound and how discrepant the ultrasound findings are with respect to your LMP. Here’s when things need to change…
If you are 8 weeks pregnant based on your LMP but you are measuring 6 ½ weeks pregnant on the ultrasound, then your due date will need to be pushed back by 1 ½ weeks (you ovulated and conceived a little later than you thought!)  Additionally, if you are 10 weeks pregnant based on your LMP but your ultrasound measurement shows you to be 11 weeks and 3 days pregnant, then your due date will be pushed up by 10 days. While we don’t expect you to do this math alone we do want you to be comfortable with the numbers and the changes that might occur. Take a look below to see when things needs to be modified:

Pregnancy Weeks based on LMP Ultrasound measurement discrepancy
Less than 9 weeks If > 5 days off, change due date
9-14 weeks If >7 days off, change due date
14-16 weeks If > 7 days off, change due date
16-22 weeks If > 10 days off, change due date
22-28 weeks If > 14 days off, change due date
28+ weeks If > 21 days off, change due date

However, a change is only permanent if confirmed by a second ultrasound. In fact, every due date needs to be confirmed twice (sort of like sending a text and an email to say we will meet at 8PM!). Even in cases where you are for-sure positive about your last menstrual period (a.k.a. LMP), we will confirm it with an ultrasound.

In cases where you don’t have a clue in the world about when you conceived, we will use two ultrasounds to create and then confirm your due date. This is particularly true for women with irregular cycles. Additionally, in pregnancies conceived after fertility treatments, we will use dates such as the day of ovulation, the day of the IUI, and the day of the ET to help us decide when the infamous D-Day (a.k.a. due date) is.

So even when you find yourself seeing a plus sign or a smiley face, don’t throw that calendar out. What you have written down, even if illegible to anyone but you, matters. It will help your OB pick your due date and know when measurements are off (say, the baby looks too small or too big). Dating is the real deal for us OBs—so make sure to keep us in the loop about those Xs and Os, no matter how hectic your schedule!