Pregnancy dating: what you need to know

pregnancy dating blog post

Pregnancy dating might seem a bit perplexing at first. Here’s how it works:

Pregnancy is conventionally dated from the first day of the last menstrual cycle. While it’s true that on this day there’s no actual pregnancy, it’s considered day 0 in the life cycle of the egg that will develop into a baby.

Pregnancy dating: regular cycles

For those with regular cycles, ovulation typically occurs 10-14 days post the onset of menses. When a period is missed and a pregnancy test turns out positive, the individual is deemed to be 4 weeks pregnant – even though the menstrual period was just 4 weeks prior. This means terms like “1-2 weeks pregnant” are not traditionally recognized; the earliest recognized pregnancy stage is at 3-4 weeks.

Pregnancy dating: irregular cycles

In the case of irregular cycles, the best method to date a pregnancy accurately is through an early pregnancy ultrasound. The prime window for such ultrasounds is between 7- and 9-weeks gestation.

Pregnancy dating and ultrasounds

But why not perform an ultrasound as soon as the first positive pregnancy test is obtained? Simply put, ultrasounds prior to the 5-week mark cannot yield precise details about the pregnancy. It’s only when the pregnancy hormone, HCG, reaches roughly 1500 that an ultrasound can identify a gestational sac within the uterus.

Here’s what ultrasounds can determine during early pregnancy:

  • 5 Weeks: Detection of the gestational sac.
  • 6 Weeks: Identification of a fetal pole and yolk sac.
  • 7 Weeks: Observation of a fetal heartbeat and its measurement.

At this stage, the developing baby is minuscule—akin to the size of a cheerio. Within this tiny entity, sonographers pinpoint a beating heart and gauge its rate. If a fetal pole measures 7mm without a detectable heartbeat, it often indicates a miscarriage.

Here’s what can be discerned from an early ultrasound:

  • Number of babies in the uterus.
  • Pregnancy’s location (ensuring it’s not ectopic).
  • Gestational age, based on the fetal pole and gestational sac sizes.
  • Fetal heartbeat.

However, some details remain elusive:

  • Placenta’s location (e.g., determining placenta previa).
  • Baby’s structural details (e.g., spotting congenital anomalies).
  • Genetic information about the baby.

These aspects can be more thoroughly evaluated during the 12-week and 20-week ultrasounds.

It’s important to remember that a due date is merely an estimated time of delivery and serves as a guide to comprehend and monitor pregnancy milestones.

Learn more about the menstrual cycle by checking out a deeper dive into the topic: My Fertility Podcast: The menstrual cycle.

My Fertility Podcast, EP02: The Menstrual Cycle

Learn more about pregnancy dating by watching this video hosted by Dr. Wais and Dr. Garcia on IG Live. You can catch session every Tuesday @9am EST. Follow Markham Fertility Centre on IG @markhamfertility