The average gestation for a singleton pregnancy is (generally) considered to be 280 days (or 40 weeks) from your last menstrual period. This method will give you the typical 40-week pregnancy. However, due dates are more complicated and less certain than that. Also, due dates after infertility or after IVF can be a little different, too. Most due dates are calculated based on the last menstrual period and assume you ovulated exactly 14 days later. But many women ovulate sooner or later. Even with fertility treatment, we don’t really know when you conceived unless you did IVF. With Clomid or intrauterine insemination (IUI), you might know when you had your trigger shot. You might know when you got a positive ovulation test result, or you might know when your basal body temperature chart indicated ovulation. But even that isn’t 100% accurate. Studies have found that ovulation can occur a few days off from what a positive ovulation test or BBT chart indicates. Also, implantation of the embryo also varies. The time of implantation can also affect how long you’ll be pregnant. You may also be surprised to learn that we don’t know for certain how long human gestation is. Primarily, this is because the moment of conception is hidden. It’s not an easy topic to study. With the exception of IVF, we have no idea when it happens. Some studies say 280 days (or 40 weeks) is the best guess for a due date. Other studies say 282 days (or 40 weeks and 2 days) is better. However, IVF due dates aren’t calculated based on fertilization day. They are calculated based on the embryo transfer date. Sometimes (in a non-IVF conceived pregnancy), due dates are adjusted or changed. This may occur after an ultrasound if the size of the fetus doesn’t match what would be expected based on the last menstrual period. If you had a Day 3 embryo transfer, your due date should be 263 days later. Use this IVF calculator to figure out your due date. However, after IVF, if a future ultrasound indicates a “wrong due date,” your due date will most likely not change. Usually, your doctor will closely monitor the pregnancy for a while to be sure the fetus is developing as expected.
History of Infertility
Women who struggled to conceive are more likely to give birth early. They are at a higher risk of premature labor, which can be threatening to your baby’s health. This is true even for singleton pregnancies. (You don’t have to have twins for your risk of premature labor to go up.) The theory is that whatever hormonal imbalances caused the infertility also impact the hormones regulating pregnancy and childbirth.
Twins or More
Most people know that the more babies growing in your womb, the more likely you’ll give birth early. Twins tend to be born earlier than singletons. Triplets tend to be born earlier than twins. Quadruplets tend to be born earlier than triplets. Your doctor may even give you two dates: a regular due date and a slightly earlier date, with the explanation that twins sometimes come sooner. This doesn’t mean your babies will be ready for the world at birth sooner, just that you shouldn’t be surprised if you go into labor earlier.
Slow or Abrupt Increase in Progesterone
The corpus luteum—which is formed from the follicle that released the egg—produces progesterone soon after ovulation. When pregnancy occurs, pregnancy hormones signal the corpus luteum to continue to produce progesterone and increase those levels. For some women, the levels rise rapidly. Others get a slow and later rise. Research has found that women with the more sluggish progesterone rise tend to give birth earlier. You might know if your progesterone has the more sluggish pattern from close monitoring in an early IVF pregnancy.
