It sounds like a scene from a TV sitcom: a couple are told when their baby is due, only for the man to whisper: “That’s impossible.”
But it turns out, your due date doesn’t always match up with when you had sex. In fact, it’s pretty common for estimated gestation not to align with conception.
Marley Hall, who’s known as Midwife Marley on social media, recently explained how this works on Instagram – and the comments are full of women who had no idea.
“I had a distressed lady contact me the other day as her husband was doubting his paternity over the pregnancy,” she said.
“Their early ultrasound scan had confirmed she was approximately five weeks pregnant but he didn’t understand how that was possible as they only had intercourse a couple of weeks earlier (they must have been apart or not had sex for some time before that). He was insisting that gestational age starts at conception.”
While in theory the man was correct – gestation does technically start at conception – it’s not the way the healthcare system records it.
“Health care providers measure pregnancies in weeks starting from the first day of the last menstrual period (LMP) .It is thought that most of us ovulate around 11-17 days after the first day of the last period,” Hall explained.
“For someone who has a an average 28 day cycle, ovulation generally happens around day 14 of their cycle, then conception probably took place about two weeks after the LMP.
“For gestational age counting, these extra two weeks are added to a pregnancy as a simple method, rather than trying to track from ovulation to fertilisation, as everyone is so different.
“When an ultrasound is performed, these extra two weeks are also accounted for when estimating the gestation of the baby.”
Hall explained that this system is helpful, particularly when someone has irregular periods, or can’t remember the dates of their last cycle.
“Estimating gestational age by LMP isn’t always accurate and although early ultrasounds can provide a good insight into the gestational age, they’re never going to be 100% accurate,” she said.
“It’s also important to note that although the worldwide average pregnancy is 40 weeks, there are many variations in full gestational lengths depending on health, height, weight, and ethnicity.”
The recording system is one of the many reasons why some women women will carry their babies until 42 weeks, and others until 38. Who knew?