When ob-gyn performs US and says "Your pregnancy is of 6 weeks" what does it mean: (1) six weeks since last menses or from actual conception date?

The question is related to some interesting story: the husband was abroad at the time of the calculated "last menses" (woman has POS with menses 2 times per year), so if he did not know the answer to this question he could think that he is not an actual father of the baby.

In addition, the women and her husband have visited ob-gyn at the exact date of calculated last menses and the doctor did not see any signs of ovulation (or was mistaken because of POS).

So, could you clarify (2) if the doctor says "the calculated date of your menses is...let's say 1 of April" does it mean that the conception was around 14 of April or around that date?

  • In Spain, when they say "your pregancy is of 6 weeks", they mean 6 weeks since last menses :) The exact time of conception can not be know. Two weeks is a good estimation, but it is not know for sure. So (1) is from last menses. (2) 2 weeks after last menses.
    – Javier
    Apr 17, 2015 at 21:52
  • 3
    I always assumed that they used that date because it was something people (usually) knew - they don't know ovulation unless they're using tests, but even a hundred years ago people could most often tell the date of their last menses. But for someone with PCOS, I'd be curious if the date of last menses is actually relevant, or if they figure out a better result.
    – Joe
    Apr 18, 2015 at 0:46
  • @Joe - they use that day only because the average menstrual cycle is 28 days, and ovulation usually occurs 14 days before the next menstrual period. If someone has 21 day cycles, ovulation usually still occurs 14 days before the next menstrual cycle, and an EDC is calculated taking this into account. Apr 18, 2015 at 2:03

3 Answers 3


The pregnancy is usually calculated from the start of the last period.

This is babycenter.com's page on 2 weeks pregnancy:

Your last period started 12 to 16 days ago, so you're probably ovulating now or will be soon. Strange as it may seem, your healthcare provider calculates your due date (and your baby's gestational age) starting from the first day of your last menstrual period.

Note that some women have cycles longer than 28 days, and some women doesn't ovulate right in the center of their cycle (Using an ovulation kit, I determined my ovulation is much closer to 3 weeks into a 4 week cycle).

For most healthcare providers, even if you know these detail, the 'start day of last period' is how they calculate, and information on what happens at different stages of the pregnancy uses this as well.


The Estimated Date of Confinement (EDC) or due date (EDD) is usually calculated from the date of the last menstrual period (LMP). With someone who has irregular periods or doesn't know the date of her LMP, it's a challenge to determine when conception occurred.

if the doctor says "the calculated date of your menses is...let's say 1 of April"...

The doctor doesn't calculate the date of the last menstrual period. The doctor doesn't need that particular piece of information if it's not available.

In that case, the treating physician must obtain what's referred to as an "ultrasound for dates". This is important mostly to know not how the baby is doing - that's available on a regular ultrasound - but for exactly how old the baby is, because towards the end of the pregnancy, the physician needs to do special tests if the pregnancy is not progressing as it should. To know what should be happening, we need to know how old the fetus is exactly. Then the doctor will give the patient her estimated due date (EDD), not the estimated date of her last period.

In other words, the doctor doesn't need to estimate LMP, only EDD, so he/she will say, "Your baby's due date is (date based on fetal age)".

Confirming the correct dates of your pregnancy. Some women are uncertain of their last menstrual period (LMP) or have irregular menstrual cycles, making it difficult for their doctor to correctly estimate when the baby is due. Establishing accurate dates can be important, especially if there are concerns about your baby later in the pregnancy (for example, if the baby is not growing well). An ultrasound in the first trimester can give an accurate estimated date of confinement (EDC) to within 3-5 days. Generally speaking, the earlier in your pregnancy the ultrasound is performed, the more accurate it will be at estimating your baby’s due date (technical factors such as the quality of the ultrasound image and the expertise of the sonographer will affect this accuracy).

For example, if a baby is not born by it's due date, the physician must start ordering non-stress tests. Since it's actually more dangerous for the baby to be born late, the physician must consider inducing labor, something that isn't in anyone's best interests if it's not necessary. So, the ultrasound for dates.

The link on the bottom of this post discusses hoe the estimated due date (EDD) is calculated. An excerpt:

Determining the first day of the LMP traditionally is the first step in establishing the EDD. By convention, the EDD is 280 days after the first day of the LMP. Because this practice assumes a regular menstrual cycle of 28 days, with ovulation occurring on the 14th day after the beginning of the menstrual cycle, this practice does not account for inaccurate recall of the LMP, irregularities in cycle length, or variability in the timing of ovulation.

If there is question of paternity, there are tests for that. But it doesn't matter if the doctor picked up signs of ovuation. What matters is if the man and woman had sex and made a baby.

Method for Estimating Due Date


Since in the current answers there is no information about normal pregnancy duration I've decided to add my own answer for the sake of clarity.

Normal pregnancy usually lasts 266 days which is exactly 38 weeks. It means the baby spends in the uterus 38 weeks.

The exact day of conception is known only in the case of artificial fertilization (i.e. IVF in vitro fertilization). Even if the couple had 1 intercourse and they know exact day of ovulation it does not mean that the conception day is equal to the day of intercourse or the date of ovulation: the actual time of meeting of the spermatozoid and the egg is later than these points and can be at the same day as well as 1-3-5 days later. So, IVF is the only case when the exact conception time can be known with absolute precise.

Because of the mentioned unpredictability, it is accepted to calculate the dates related to a pregnancy based on the date of the last menstruation. This is why most people think that normal pregnancy lasts 40 weeks.

Ironically speaking, the mentioned explanation "means" that at the time of an ovulation a woman is already 2 weeks "pregnant".

If this date is unknown, the US can help to calculate this date in order to provide others points (date of delivery etc). Thus, when ob-gyn says "X weeks pregnancy" it means "the duration is based on the last menses date which in turn was calculated by the formula inside the US machine - this formula adds 14 days to the calculated actual duration of pregnancy.

Back to question, it is clear that the couple should calculate the conception date adding (around) 14 days to the calculated last menses.

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